diff --git a/src/libCom/cvtFast.c b/src/libCom/cvtFast.c
index 38c82e41a..090232e2e 100644
--- a/src/libCom/cvtFast.c
+++ b/src/libCom/cvtFast.c
@@ -514,12 +514,13 @@ int epicsShareAPI cvtLongToHexString(
     }
     if(source<0) {
 	if(source == LONG_MIN) {
-	    sprintf(pdest,"%lx",LONG_MIN);
+	    sprintf(pdest,"-0x%lx",LONG_MIN);
 	    return((int)strlen(pdest));
 	}
 	*pdest++ = '-';
 	source = -source;
     }
+    *pdest++ = '0'; *pdest++ = 'x';
     val = source;
     for(i=0; val!=0; i++) {
 	temp = val/16;
@@ -550,12 +551,13 @@ int epicsShareAPI cvtLongToOctalString(
     }
     if(source<0) {
 	if(source == LONG_MIN) {
-	    sprintf(pdest,"%lo",LONG_MIN);
+	    sprintf(pdest,"-0%lo",LONG_MIN);
 	    return((int)strlen(pdest));
 	}
 	*pdest++ = '-';
 	source = -source;
     }
+    *pdest++ = '0';
     val = source;
     for(i=0; val!=0; i++) {
 	temp = val/8;