diff --git a/src/cpu/core_16/prefix_66_of.h b/src/cpu/core_16/prefix_66_of.h
index 676f0007..c7b0f81f 100644
--- a/src/cpu/core_16/prefix_66_of.h
+++ b/src/cpu/core_16/prefix_66_of.h
@@ -79,6 +79,8 @@ switch (Fetchb()) {
 					if (flags.cf) *eard&=~mask;
 					else *eard|=mask;
 					break;
+				default:
+					E_Exit("CPU:66:0F:BA:Illegal subfunction %X",rm & 0x38);
 				}
 			} else {
 				GetEAa;Bit32u old=LoadMd(eaa);
@@ -98,6 +100,8 @@ switch (Fetchb()) {
 					else old|=mask;
 					SaveMd(eaa,old);
 					break;
+				default:
+					E_Exit("CPU:66:0F:BA:Illegal subfunction %X",rm & 0x38);
 				}
 			}
 			if (flags.type!=t_CF) flags.prev_type=flags.type;
diff --git a/src/cpu/core_16/prefix_of.h b/src/cpu/core_16/prefix_of.h
index 3decea1f..3e3a4b4b 100644
--- a/src/cpu/core_16/prefix_of.h
+++ b/src/cpu/core_16/prefix_of.h
@@ -314,6 +314,8 @@ switch(Fetchb()) {
 				case 0x38:									/* BTC */
 					*earw^=mask;
 					break;
+				default:
+					E_Exit("CPU:0F:BA:Illegal subfunction %X",rm & 0x38);
 				}
 			} else {
 				GetEAa;Bit16u old=LoadMw(eaa);
@@ -330,6 +332,8 @@ switch(Fetchb()) {
 				case 0x38:									/* BTC */
 					SaveMw(eaa,old ^ mask);
 					break;
+				default:
+					E_Exit("CPU:0F:BA:Illegal subfunction %X",rm & 0x38);
 				}
 			}
 			if (flags.type!=t_CF)	{ flags.prev_type=flags.type;flags.type=t_CF;	}