diff --git a/developer/src/common/include/kmn_compiler_errors.h b/developer/src/common/include/kmn_compiler_errors.h
index 4973f58abe..78ab21efa1 100644
--- a/developer/src/common/include/kmn_compiler_errors.h
+++ b/developer/src/common/include/kmn_compiler_errors.h
@@ -179,6 +179,7 @@
 #define CERR_ExtendedStringTooLong                         0x00004076
 #define CERR_VirtualKeyExpansionTooLong                    0x00004077
 #define CERR_CharacterRangeTooLong                         0x00004078
+#define CERR_NonBMPCharactersNotSupportedInKeySection      0x00004079
 
 #define CWARN_TooManyWarnings                              0x00002080
 #define CWARN_OldVersion                                   0x00002081
diff --git a/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts b/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
index 32dac9fb66..ec7c81a7a8 100644
--- a/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
+++ b/developer/src/kmc-kmn/src/compiler/kmn-compiler-messages.ts
@@ -571,6 +571,8 @@ export class KmnCompilerMessages {
   static ERROR_CharacterRangeTooLong                          = SevError | 0x078;
   static Error_CharacterRangeTooLong                          = () => m(this.ERROR_CharacterRangeTooLong, `Character range is too large and cannot be expanded`);
 
+  static ERROR_NonBMPCharactersNotSupportedInKeySection       = SevError | 0x079;
+  static Error_NonBMPCharactersNotSupportedInKeySection       = () => m(this.ERROR_NonBMPCharactersNotSupportedInKeySection, `Characters with codepoints over U+FFFF are not supported in the key part of the rule`);
 
   static WARN_TooManyWarnings                                 = SevWarn | 0x080;
   static Warn_TooManyWarnings                                 = () => m(this.WARN_TooManyWarnings, `Too many warnings or errors`);
diff --git a/developer/src/kmcmplib/src/CompMsg.cpp b/developer/src/kmcmplib/src/CompMsg.cpp
index 86a9b4d475..e075a4c808 100644
--- a/developer/src/kmcmplib/src/CompMsg.cpp
+++ b/developer/src/kmcmplib/src/CompMsg.cpp
@@ -115,6 +115,7 @@ const struct CompilerError CompilerErrors[] = {
     { CERR_ExtendedStringTooLong                         , "Extended string is too long" },
     { CERR_VirtualKeyExpansionTooLong                    , "Virtual key expansion is too large" },
     { CERR_CharacterRangeTooLong                         , "Character range is too large and cannot be expanded" },
+    { CERR_NonBMPCharactersNotSupportedInKeySection      , "Characters with codepoints over U+FFFF are not supported in the key part of the rule" },
 
     { CHINT_UnreachableRule                              , "This rule will never be matched as another rule takes precedence"},
     { CHINT_NonUnicodeFile                               , "Keyman Developer has detected that the file has ANSI encoding. Consider converting this file to UTF-8"},
diff --git a/developer/src/kmcmplib/src/Compiler.cpp b/developer/src/kmcmplib/src/Compiler.cpp
index 8d0f11af8f..b014d8d7c8 100644
--- a/developer/src/kmcmplib/src/Compiler.cpp
+++ b/developer/src/kmcmplib/src/Compiler.cpp
@@ -1210,7 +1210,7 @@ int GetCompileTargetsFromTargetsStore(const KMX_WCHAR* store) {
 KMX_BOOL IsValidKeyboardVersion(KMX_WCHAR *dpString) {   // I4140
   /**
     version format: /^\d+(\.\d+)*$/
-    e.g. 9.0.3, 1.0, 1.2.3.4, 6.2.1.4.6.4, 11.22.3 are all ok; 
+    e.g. 9.0.3, 1.0, 1.2.3.4, 6.2.1.4.6.4, 11.22.3 are all ok;
     empty string is not permitted; whitespace is not permitted
   */
 
@@ -1471,7 +1471,14 @@ KMX_DWORD ProcessKeyLineImpl(PFILE_KEYBOARD fk, PKMX_WCHAR str, KMX_BOOL IsUnico
 
     str = p + 1;
     if ((msg = GetXString(fk, str, u">", pklKey, GLOBAL_BUFSIZE - 1, (int)(str - pp), &p, TRUE, IsUnicode)) != CERR_None) return msg;
+
     if (pklKey[0] == 0) return CERR_ZeroLengthString;
+
+    if(Uni_IsSurrogate1(pklKey[0])) {
+      // #11643: non-BMP characters do not makes sense for key codes
+      return CERR_NonBMPCharactersNotSupportedInKeySection;
+    }
+
     if (xstrlen(pklKey) > 1) AddWarning(CWARN_KeyBadLength);
   } else {
     if ((msg = GetXString(fk, str, u">", pklIn, GLOBAL_BUFSIZE - 1, (int)(str - pp), &p, TRUE, IsUnicode)) != CERR_None) return msg;
@@ -1653,9 +1660,10 @@ KMX_DWORD ExpandKp(PFILE_KEYBOARD fk, PFILE_KEY kpp, KMX_DWORD storeIndex)
       default:
         return CERR_CodeInvalidInKeyStore;
       }
-    }
-    else
-    {
+    } else if(Uni_IsSurrogate1(*pn)) {
+      // #11643: non-BMP characters do not makes sense for key codes
+      return CERR_NonBMPCharactersNotSupportedInKeySection;
+    } else {
       k->Key = *pn;				// set the key to store offset.
       k->ShiftFlags = 0;
     }