diff --git a/lib/libc/stdlib/strtoll.c b/lib/libc/stdlib/strtoll.c
index b31cca4ab199..a5720df447d4 100644
--- a/lib/libc/stdlib/strtoll.c
+++ b/lib/libc/stdlib/strtoll.c
@@ -56,7 +56,7 @@ strtoq(nptr, endptr, base)
 {
 	register const char *s;
 	register u_quad_t acc;
-	register int c;
+	register unsigned char c;
 	register u_quad_t qbase, cutoff;
 	register int neg, any, cutlim;
 
@@ -109,6 +109,8 @@ strtoq(nptr, endptr, base)
 	cutlim = cutoff % qbase;
 	cutoff /= qbase;
 	for (acc = 0, any = 0;; c = *s++) {
+		if (!isascii(c))
+			break;
 		if (isdigit(c))
 			c -= '0';
 		else if (isalpha(c))
diff --git a/lib/libc/stdlib/strtoq.c b/lib/libc/stdlib/strtoq.c
index b31cca4ab199..a5720df447d4 100644
--- a/lib/libc/stdlib/strtoq.c
+++ b/lib/libc/stdlib/strtoq.c
@@ -56,7 +56,7 @@ strtoq(nptr, endptr, base)
 {
 	register const char *s;
 	register u_quad_t acc;
-	register int c;
+	register unsigned char c;
 	register u_quad_t qbase, cutoff;
 	register int neg, any, cutlim;
 
@@ -109,6 +109,8 @@ strtoq(nptr, endptr, base)
 	cutlim = cutoff % qbase;
 	cutoff /= qbase;
 	for (acc = 0, any = 0;; c = *s++) {
+		if (!isascii(c))
+			break;
 		if (isdigit(c))
 			c -= '0';
 		else if (isalpha(c))
diff --git a/lib/libc/stdlib/strtoul.c b/lib/libc/stdlib/strtoul.c
index e60556c0aa50..0bf6b82ef085 100644
--- a/lib/libc/stdlib/strtoul.c
+++ b/lib/libc/stdlib/strtoul.c
@@ -54,7 +54,7 @@ strtoul(nptr, endptr, base)
 {
 	register const char *s = nptr;
 	register unsigned long acc;
-	register int c;
+	register unsigned char c;
 	register unsigned long cutoff;
 	register int neg = 0, any, cutlim;
 
@@ -80,6 +80,8 @@ strtoul(nptr, endptr, base)
 	cutoff = (unsigned long)ULONG_MAX / (unsigned long)base;
 	cutlim = (unsigned long)ULONG_MAX % (unsigned long)base;
 	for (acc = 0, any = 0;; c = *s++) {
+		if (!isascii(c))
+			break;
 		if (isdigit(c))
 			c -= '0';
 		else if (isalpha(c))
diff --git a/lib/libc/stdlib/strtoull.c b/lib/libc/stdlib/strtoull.c
index ee1e13b7d7c2..16dda0be3971 100644
--- a/lib/libc/stdlib/strtoull.c
+++ b/lib/libc/stdlib/strtoull.c
@@ -56,7 +56,7 @@ strtouq(nptr, endptr, base)
 {
 	register const char *s = nptr;
 	register u_quad_t acc;
-	register int c;
+	register unsigned char c;
 	register u_quad_t qbase, cutoff;
 	register int neg, any, cutlim;
 
@@ -87,6 +87,8 @@ strtouq(nptr, endptr, base)
 	cutoff = (u_quad_t)UQUAD_MAX / qbase;
 	cutlim = (u_quad_t)UQUAD_MAX % qbase;
 	for (acc = 0, any = 0;; c = *s++) {
+		if (!isascii(c))
+			break;
 		if (isdigit(c))
 			c -= '0';
 		else if (isalpha(c))
diff --git a/lib/libc/stdlib/strtouq.c b/lib/libc/stdlib/strtouq.c
index ee1e13b7d7c2..16dda0be3971 100644
--- a/lib/libc/stdlib/strtouq.c
+++ b/lib/libc/stdlib/strtouq.c
@@ -56,7 +56,7 @@ strtouq(nptr, endptr, base)
 {
 	register const char *s = nptr;
 	register u_quad_t acc;
-	register int c;
+	register unsigned char c;
 	register u_quad_t qbase, cutoff;
 	register int neg, any, cutlim;
 
@@ -87,6 +87,8 @@ strtouq(nptr, endptr, base)
 	cutoff = (u_quad_t)UQUAD_MAX / qbase;
 	cutlim = (u_quad_t)UQUAD_MAX % qbase;
 	for (acc = 0, any = 0;; c = *s++) {
+		if (!isascii(c))
+			break;
 		if (isdigit(c))
 			c -= '0';
 		else if (isalpha(c))