diff --git a/src/libCom/calc/calcPerform.c b/src/libCom/calc/calcPerform.c
index 117ed7c9a..f2a364ff4 100644
--- a/src/libCom/calc/calcPerform.c
+++ b/src/libCom/calc/calcPerform.c
@@ -42,14 +42,12 @@ epicsShareFunc long
     double stack[CALCPERFORM_STACK+1];	/* zero'th entry not used */
     double *ptop;			/* stack pointer */
     double top; 			/* value from top of stack */
-    int itop;				/* integer from top of stack */
-    char nargs;
+    long itop;				/* integer from top of stack */
+    int nargs;
 
     /* initialize */
     ptop = stack;
 
-    if(*pinst == END_EXPRESSION) return -1;
-
     /* RPN evaluation loop */
     while (*pinst != END_EXPRESSION){
 	switch (*pinst){
@@ -132,8 +130,11 @@ epicsShareFunc long
 	    break;
 
 	case MODULO:
-	    top = *ptop--;
-	    *ptop = fmod(*ptop, top);
+	    itop = (long) *ptop--;
+	    if (itop)
+		*ptop = (long) *ptop % itop;
+	    else
+		*ptop = 0.0 / itop;   /* NaN */
 	    break;
 
 	case POWER:
@@ -276,33 +277,33 @@ epicsShareFunc long
 	    break;
 
 	case BIT_OR:
-	    itop = (int) *ptop--;
-	    *ptop = (int) *ptop | itop;
+	    itop = (long) *ptop--;
+	    *ptop = (long) *ptop | itop;
 	    break;
 
 	case BIT_AND:
-	    itop = (int) *ptop--;
-	    *ptop = (int) *ptop & itop;
+	    itop = (long) *ptop--;
+	    *ptop = (long) *ptop & itop;
 	    break;
 
 	case BIT_EXCL_OR:
-	    itop = (int) *ptop--;
-	    *ptop = (int) *ptop ^ itop;
+	    itop = (long) *ptop--;
+	    *ptop = (long) *ptop ^ itop;
 	    break;
 
 	case BIT_NOT:
-	    itop = (int) *ptop;
+	    itop = (long) *ptop;
 	    *ptop = ~itop;
 	    break;
 
 	case RIGHT_SHIFT:
-	    itop = (int) *ptop--;
-	    *ptop = (int) *ptop >> itop;
+	    itop = (long) *ptop--;
+	    *ptop = (long) *ptop >> itop;
 	    break;
 
 	case LEFT_SHIFT:
-	    itop = (int) *ptop--;
-	    *ptop = (int) *ptop << itop;
+	    itop = (long) *ptop--;
+	    *ptop = (long) *ptop << itop;
 	    break;
 
 	case NOT_EQ:
diff --git a/src/libCom/test/epicsCalcTest.cpp b/src/libCom/test/epicsCalcTest.cpp
index 9cc2b28ce..5b8081369 100644
--- a/src/libCom/test/epicsCalcTest.cpp
+++ b/src/libCom/test/epicsCalcTest.cpp
@@ -200,7 +200,7 @@ MAIN(epicsCalcTest)
     Inf /= NaN;
     NaN /= NaN;
     
-    testPlan(532);
+    testPlan(533);
     
     /* LITERAL_OPERAND elements */
     testExpr(0);
@@ -467,6 +467,7 @@ MAIN(epicsCalcTest)
     testExpr(7 % 4);
     testExpr(-7 % 4);
     testExpr(63 % 16 % 6)
+    testCalc("1 % 0", NaN);
     
     testExpr(7 & 4);