diff --git a/src/ca65/instr.c b/src/ca65/instr.c
index dd1729ec3..7d80bd74c 100644
--- a/src/ca65/instr.c
+++ b/src/ca65/instr.c
@@ -887,7 +887,14 @@ int FindInstruction (const char* Ident)
 
     /* Make a copy, and uppercase that copy */
     I = 0;
-    while (Ident[I]) {
+    while (Ident[I] != '\0') {
+        /* If the identifier is longer than the longest mnemonic, it cannot
+         * be one.
+         */
+        if (I >= sizeof (Key) - 1) {
+            /* Not found, no need for further action */
+            return -1;
+        }
         Key[I] = toupper ((unsigned char)Ident[I]);
         ++I;
     }
diff --git a/src/ca65/instr.h b/src/ca65/instr.h
index 20f957203..7429e447c 100644
--- a/src/ca65/instr.h
+++ b/src/ca65/instr.h
@@ -97,7 +97,7 @@
 /* Description for one instruction */
 typedef struct InsDesc InsDesc;
 struct InsDesc {
-    char       	      	Mnemonic [8];
+    char       	      	Mnemonic[5];
     unsigned long      	AddrMode;	    	/* Valid adressing modes */
     unsigned char     	BaseCode;  	    	/* Base opcode */
     unsigned char     	ExtCode;   	 	/* Number of ext code table */