Index: ext/session/sqlite3session.c
==================================================================
--- ext/session/sqlite3session.c
+++ ext/session/sqlite3session.c
@@ -1622,11 +1622,13 @@
           }
         }
       }
       sqlite3_free((char*)azCol);
       if( bMismatch ){
-        *pzErrMsg = sqlite3_mprintf("table schemas do not match");
+        if( pzErrMsg ){
+          *pzErrMsg = sqlite3_mprintf("table schemas do not match");
+        }
         rc = SQLITE_SCHEMA;
       }
       if( bHasPk==0 ){
         /* Ignore tables with no primary keys */
         goto diff_out;