diff --git a/lib/libc/net/gethostnamadr.c b/lib/libc/net/gethostnamadr.c
index 8e02d2c103d..010e4db5d95 100644
--- a/lib/libc/net/gethostnamadr.c
+++ b/lib/libc/net/gethostnamadr.c
@@ -103,7 +103,7 @@ init_services()
 			if (cp == NULL)
 				continue;
 			do {
-				if (isalpha(cp[0])) {
+				if (isalpha((unsigned char)cp[0])) {
 					service_order[cc] = get_service_name(cp);
 					if(service_order[cc] != SERVICE_NONE)
 						cc++;
diff --git a/lib/libc/net/getnetnamadr.c b/lib/libc/net/getnetnamadr.c
index 3ac10c2cbaa..cb5871b80b0 100644
--- a/lib/libc/net/getnetnamadr.c
+++ b/lib/libc/net/getnetnamadr.c
@@ -101,7 +101,7 @@ init_services()
 			if (cp == NULL)
 				continue;
 			do {
-				if (isalpha(cp[0])) {
+				if (isalpha((unsigned char)cp[0])) {
 					service_order[cc] = get_service_name(cp);
 					if(service_order[cc] != SERVICE_NONE)
 						cc++;