diff --git a/safeclib/stpcpy_s.c b/safeclib/stpcpy_s.c
index 8e70f04..c30b78c 100644
--- a/safeclib/stpcpy_s.c
+++ b/safeclib/stpcpy_s.c
@@ -157,7 +157,9 @@ stpcpy_s(char *dest, rsize_t dmax, const char *src, errno_t *err)
     		if (*dest == '\0') {
 #ifdef SAFECLIB_STR_NULL_SLACK
                 /* null slack to clear any data */
-    		    while (dmax) { *dest = '\0'; dmax--; dest++; }
+		    char *filler = dest;
+
+    		    while (dmax) { *filler = '\0'; dmax--; filler++; }
 #endif
     		    *err = RCNEGATE(EOK);
     		    return dest;
@@ -191,7 +193,9 @@ stpcpy_s(char *dest, rsize_t dmax, const char *src, errno_t *err)
             if (*dest == '\0') {
 #ifdef SAFECLIB_STR_NULL_SLACK
                 /* null slack to clear any data */
-                while (dmax) { *dest = '\0'; dmax--; dest++; }
+                char *filler = dest;
+
+                while (dmax) { *filler = '\0'; dmax--; filler++; }
 #endif
                 *err = RCNEGATE(EOK);
                 return dest;
@@ -219,7 +223,9 @@ stpcpy_s(char *dest, rsize_t dmax, const char *src, errno_t *err)
             if (*dest == '\0') {
 #ifdef SAFECLIB_STR_NULL_SLACK
                 /* null slack to clear any data */
-                while (dmax) { *dest = '\0'; dmax--; dest++; }
+                char *filler = dest;
+
+                while (dmax) { *filler = '\0'; dmax--; filler++; }
 #endif
                 *err = RCNEGATE(EOK);
                 return dest;
diff --git a/safeclib/stpncpy_s.c b/safeclib/stpncpy_s.c
index 212d116..a52b9d2 100644
--- a/safeclib/stpncpy_s.c
+++ b/safeclib/stpncpy_s.c
@@ -185,7 +185,7 @@ stpncpy_s(char *dest, rsize_t dmax, const char *src, rsize_t smax, errno_t *err)
 
 #ifdef SAFECLIB_STR_NULL_SLACK
     /* dmwheel1: Add check to prevent destruction of overlap into destination */
-    if ((src < dest) && ((src+dmax) >= dest)) {
+    if ((src < dest) && ((src + smax) > dest)) {
         invoke_safe_str_constraint_handler("stpncpy_s: src+dmax overlaps into dest",
                    NULL, ESOVRLP);
         *err = RCNEGATE(ESOVRLP);
@@ -193,7 +193,7 @@ stpncpy_s(char *dest, rsize_t dmax, const char *src, rsize_t smax, errno_t *err)
     }
 
     /* dmwheel1: Add check to prevent destruction of overlap into source */
-    if ((dest < src) && ((dest+dmax) >= src)) {
+    if ((dest < src) && ((dest + dmax) > src)) {
         invoke_safe_str_constraint_handler("stpncpy_s: dest+dmax overlaps into src",
                    NULL, ESOVRLP);
         *err = RCNEGATE(ESOVRLP);
diff --git a/safeclib/wcpcpy_s.c b/safeclib/wcpcpy_s.c
index e4cc9f6..aea6555 100644
--- a/safeclib/wcpcpy_s.c
+++ b/safeclib/wcpcpy_s.c
@@ -178,7 +178,9 @@ wcpcpy_s(wchar_t* dest, rsize_t dmax, const wchar_t* src, errno_t *err)
             if (*dest == L'\0') {
 #ifdef SAFECLIB_STR_NULL_SLACK
                 /* null slack to clear any data */
-                while (dmax) { *dest = L'\0'; dmax--; dest++; }
+                wchar_t *filler = dest;
+
+                while (dmax) { *filler = L'\0'; dmax--; filler++; }
 #endif
                 *err = RCNEGATE(EOK);
                 return dest; /* successful return */
@@ -204,7 +206,9 @@ wcpcpy_s(wchar_t* dest, rsize_t dmax, const wchar_t* src, errno_t *err)
             if (*dest == L'\0') {
 #ifdef SAFECLIB_STR_NULL_SLACK
                 /* null slack to clear any data */
-                while (dmax) { *dest = L'\0'; dmax--; dest++; }
+                wchar_t *filler = dest;
+
+                while (dmax) { *filler = L'\0'; dmax--; filler++; }
 #endif
                 *err = RCNEGATE(EOK);
                 return dest; /* successful return */
diff --git a/unittests/test_stpncpy_s.c b/unittests/test_stpncpy_s.c
index 7184b59..5580979 100644
--- a/unittests/test_stpncpy_s.c
+++ b/unittests/test_stpncpy_s.c
@@ -428,7 +428,12 @@ printf("Test #%d:\n", ++testno);
 		}
 		/* be sure that the slack is correct */
 		for (i=1; i<6; i++) {
-			if (ret[i] != 'x') {
+#ifdef SAFECLIB_STR_NULL_SLACK
+			const char slack = '\0';
+#else
+			const char slack = 'x';
+#endif // SAFECLIB_STR_NULL_SLACK
+			if (ret[i] != slack) {
 				printf("%s %u Incorrect Slack at returned ptr index %d  Error rc=%u \n",
 									 __FUNCTION__, __LINE__,  i, rc );
 				++errs;
@@ -473,7 +478,12 @@ printf("Test #%d:\n", ++testno);
 			}
 			/* be sure that the slack is correct */
 			for (; i<15; i++) {
-				if (ret[i] != 'x') {
+#ifdef SAFECLIB_STR_NULL_SLACK
+				const char slack = '\0';
+#else
+				const char slack = 'x';
+#endif // SAFECLIB_STR_NULL_SLACK
+				if (ret[i] != slack) {
 					printf("%s %u Incorrect Slack at returned ptr index %d  Error rc=%u \n",
 										 __FUNCTION__, __LINE__,  i, rc );
 					++errs;
@@ -511,7 +521,12 @@ printf("Test #%d:\n", ++testno);
 		} else {
 			/* be sure that the slack is correct */
 			for (i=1; i<5; i++) {
-				if (ret[i] != 'x') {
+#ifdef SAFECLIB_STR_NULL_SLACK
+				const char slack = '\0';
+#else
+				const char slack = 'x';
+#endif // SAFECLIB_STR_NULL_SLACK
+				if (ret[i] != slack) {
 					printf("%s %u Incorrect Slack at returned ptr index %d  Error rc=%u \n",
 										 __FUNCTION__, __LINE__,  i, rc );
 					++errs;
@@ -549,7 +564,12 @@ printf("Test #%d:\n", ++testno);
 		} else {
 			/* be sure that the slack is correct */
 			for (i=1; i<5; i++) {
-				if (ret[i] != 'x') {
+#ifdef SAFECLIB_STR_NULL_SLACK
+				const char slack = '\0';
+#else
+				const char slack = 'x';
+#endif // SAFECLIB_STR_NULL_SLACK
+				if (ret[i] != slack) {
 					printf("%s %u Incorrect Slack at returned ptr index %d  Error rc=%u \n",
 										 __FUNCTION__, __LINE__,  i, rc );
 					++errs;