diff --git a/src/ZB.MOM.WW.ScadaBridge.Host/Program.cs b/src/ZB.MOM.WW.ScadaBridge.Host/Program.cs
index 9a766a4d..8eb7deff 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Host/Program.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Host/Program.cs
@@ -60,18 +60,19 @@ var configuration = new ConfigurationBuilder()
 // scadaproj#4) — an expander left on SQLite there would resolve pre-host ${secret:} references
 // from a stale/empty local store, silently diverging from what the node's own hub serves. Every
 // other case (sites, SqlServer mode, replication off) keeps the local SQLite path exactly as it
-// always was. Central+Grpc with a BLANK connection string deliberately falls through to the
-// SQLite path too: that boot is about to fail in AddScadaBridgeSecrets with the message naming
-// Secrets:SqlServer:ConnectionString, so the throw is not duplicated here.
+// always was. A blank or ${secret:}-valued connection string fails HERE, through the same
+// EnsureCentralSharedStoreConnectionString check AddScadaBridgeSecrets runs — this is the
+// earliest code that would otherwise hand the bad value to SqlConnection, whose generic
+// "initialization string" format error would bury the designed message.
 var expanderUsesSharedSqlStore =
     string.Equals(
         configuration["ScadaBridge:Node:Role"], "Central", StringComparison.OrdinalIgnoreCase)
-    && SecretsRegistration.UsesGrpcHub(configuration)
-    && !string.IsNullOrWhiteSpace(configuration[SecretsRegistration.HubConnectionStringKey]);
+    && SecretsRegistration.UsesGrpcHub(configuration);
 
 var expanderServices = new ServiceCollection();
 if (expanderUsesSharedSqlStore)
 {
+    SecretsRegistration.EnsureCentralSharedStoreConnectionString(configuration);
     expanderServices.AddZbSecretsSqlServerStore(configuration, "Secrets");
 }
 else
diff --git a/src/ZB.MOM.WW.ScadaBridge.Host/SecretsRegistration.cs b/src/ZB.MOM.WW.ScadaBridge.Host/SecretsRegistration.cs
index ddcbe189..9ecb761a 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Host/SecretsRegistration.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Host/SecretsRegistration.cs
@@ -185,6 +185,59 @@ public static class SecretsRegistration
             && ResolveReplicationMode(config) == SecretsReplicationMode.Grpc;
     }
 
+    /// <summary>
+    /// The one definition of "a Central node in Grpc mode has a usable shared-store connection
+    /// string", shared by <see cref="AddScadaBridgeSecrets"/> and the Layer-A expander in
+    /// <c>Program.cs</c> so the two can never diverge. The expander runs FIRST on a real boot —
+    /// without this sharing, a bad value would die inside <c>SqlConnection</c> with a generic
+    /// "initialization string" format error before the designed message ever fired.
+    /// </summary>
+    /// <param name="config">Application configuration.</param>
+    /// <returns>The validated connection string.</returns>
+    /// <exception cref="InvalidOperationException">
+    /// <see cref="HubConnectionStringKey"/> is blank or contains a <c>${secret:}</c> reference.
+    /// </exception>
+    internal static string EnsureCentralSharedStoreConnectionString(IConfiguration config)
+    {
+        var hubConnectionString = config[HubConnectionStringKey];
+
+        if (string.IsNullOrWhiteSpace(hubConnectionString))
+        {
+            // Fail closed, matching the mode's philosophy: central in Grpc mode REQUIRES the
+            // shared SQL store. An independent local store per central node is exactly the
+            // divergence scadaproj#4 recorded — one central hub answered authenticated followers
+            // with an EMPTY manifest, so a hub failover would "succeed" against nothing and stop
+            // convergence silently.
+            throw new InvalidOperationException(
+                $"{HubConnectionStringKey} is empty, but this node is Central with "
+                    + $"{ReplicationModeKey}=Grpc. Central in Grpc mode requires the "
+                    + "SHARED SQL-Server secret store — an independent local store per "
+                    + "central node is the divergence scadaproj#4 recorded (a hub "
+                    + "failover would 'succeed' against an empty manifest). Supply the "
+                    + "connection string via appsettings or the environment "
+                    + "(Secrets__SqlServer__ConnectionString).");
+        }
+
+        if (hubConnectionString.Contains("${secret:", StringComparison.Ordinal))
+        {
+            // The secrets store's own connection string can never be a secret reference: the
+            // pre-host expander needs the store this string points at in order to resolve it —
+            // bootstrap circularity, the same rule the class remarks document for the hub bearer
+            // token. The message deliberately does not echo the configured value: a mistyped
+            // value here could be a pasted real credential.
+            throw new InvalidOperationException(
+                $"{HubConnectionStringKey} contains a ${{secret:}} reference. The "
+                    + "secret store's own connection string can never be a secret "
+                    + "reference — the pre-host expander needs the store this string "
+                    + "points at in order to resolve it (bootstrap circularity, the "
+                    + "same rule as Secrets:GrpcHub:BearerToken). Supply a literal "
+                    + "value via appsettings or the environment "
+                    + "(Secrets__SqlServer__ConnectionString).");
+        }
+
+        return hubConnectionString;
+    }
+
     /// <summary>
     /// Registers the host container's secret store: a plain local SQLite store by default, or a
     /// replicating topology when <see cref="ReplicationEnabledKey"/> is set — a shared SQL-Server
@@ -212,45 +265,11 @@ public static class SecretsRegistration
             switch (role)
             {
                 case SecretsNodeRole.Central:
-                    // Both pre-checks run BEFORE any registration so the boot failure names the
-                    // real problem instead of surfacing later as a package validation message or a
-                    // malformed-connection-string fault on first use.
-                    var hubConnectionString = config[HubConnectionStringKey];
-
-                    if (string.IsNullOrWhiteSpace(hubConnectionString))
-                    {
-                        // Fail closed, matching the mode's philosophy: central in Grpc mode
-                        // REQUIRES the shared SQL store. An independent local store per central
-                        // node is exactly the divergence scadaproj#4 recorded — one central hub
-                        // answered authenticated followers with an EMPTY manifest, so a hub
-                        // failover would "succeed" against nothing and stop convergence silently.
-                        throw new InvalidOperationException(
-                            $"{HubConnectionStringKey} is empty, but this node is Central with "
-                                + $"{ReplicationModeKey}=Grpc. Central in Grpc mode requires the "
-                                + "SHARED SQL-Server secret store — an independent local store per "
-                                + "central node is the divergence scadaproj#4 recorded (a hub "
-                                + "failover would 'succeed' against an empty manifest). Supply the "
-                                + "connection string via appsettings or the environment "
-                                + "(Secrets__SqlServer__ConnectionString).");
-                    }
-
-                    if (hubConnectionString.Contains("${secret:", StringComparison.Ordinal))
-                    {
-                        // The secrets store's own connection string can never be a secret
-                        // reference: the pre-host expander needs the store this string points at
-                        // in order to resolve it — bootstrap circularity, the same rule the class
-                        // remarks document for the hub bearer token. The message deliberately
-                        // does not echo the configured value: a mistyped value here could be a
-                        // pasted real credential.
-                        throw new InvalidOperationException(
-                            $"{HubConnectionStringKey} contains a ${{secret:}} reference. The "
-                                + "secret store's own connection string can never be a secret "
-                                + "reference — the pre-host expander needs the store this string "
-                                + "points at in order to resolve it (bootstrap circularity, the "
-                                + "same rule as Secrets:GrpcHub:BearerToken). Supply a literal "
-                                + "value via appsettings or the environment "
-                                + "(Secrets__SqlServer__ConnectionString).");
-                    }
+                    // Runs BEFORE any registration so the boot failure names the real problem
+                    // instead of surfacing later as a package validation message or a
+                    // malformed-connection-string fault on first use. On a real boot the Layer-A
+                    // expander in Program.cs has already run the same check even earlier.
+                    EnsureCentralSharedStoreConnectionString(config);
 
                     // Central's store is the SHARED SQL-Server store: registers
                     // SqlServerSecretStore as ISecretStore BEFORE calling AddZbSecrets internally,