diff --git a/fusee/fusee-primary/src/lib/vsprintf.c b/fusee/fusee-primary/src/lib/vsprintf.c
index 0dd4c4514..b356b443a 100644
--- a/fusee/fusee-primary/src/lib/vsprintf.c
+++ b/fusee/fusee-primary/src/lib/vsprintf.c
@@ -508,7 +508,7 @@ char *uuid_string(char *buf, char *end, const uint8_t *addr,
 	if (uc) {
 		p = uuid;
 		do {
-			*p = toupper(*p);
+			*p = toupper((unsigned char)*p);
 		} while (*(++p));
 	}
 
@@ -1177,7 +1177,7 @@ do {									\
 		case FORMAT_TYPE_PTR:
 			save_arg(void *);
 			/* skip all alphanumeric pointer suffixes */
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
@@ -1340,7 +1340,7 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const uint32_t *bin_buf
 
 		case FORMAT_TYPE_PTR:
 			str = pointer(fmt+1, str, end, get_arg(void *), spec);
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
diff --git a/sept/sept-secondary/src/lib/vsprintf.c b/sept/sept-secondary/src/lib/vsprintf.c
index e0393f4c0..b356b443a 100644
--- a/sept/sept-secondary/src/lib/vsprintf.c
+++ b/sept/sept-secondary/src/lib/vsprintf.c
@@ -508,7 +508,7 @@ char *uuid_string(char *buf, char *end, const uint8_t *addr,
 	if (uc) {
 		p = uuid;
 		do {
-			*p = toupper(*p);
+			*p = toupper((unsigned char)*p);
 		} while (*(++p));
 	}
 
@@ -882,7 +882,7 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 		case FORMAT_TYPE_PTR:
 			str = pointer(fmt+1, str, end, va_arg(args, void *),
 				      spec);
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
@@ -1177,7 +1177,7 @@ do {									\
 		case FORMAT_TYPE_PTR:
 			save_arg(void *);
 			/* skip all alphanumeric pointer suffixes */
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
@@ -1340,7 +1340,7 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const uint32_t *bin_buf
 
 		case FORMAT_TYPE_PTR:
 			str = pointer(fmt+1, str, end, get_arg(void *), spec);
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
diff --git a/thermosphere/src/lib/vsprintf.c b/thermosphere/src/lib/vsprintf.c
index e0393f4c0..b356b443a 100644
--- a/thermosphere/src/lib/vsprintf.c
+++ b/thermosphere/src/lib/vsprintf.c
@@ -508,7 +508,7 @@ char *uuid_string(char *buf, char *end, const uint8_t *addr,
 	if (uc) {
 		p = uuid;
 		do {
-			*p = toupper(*p);
+			*p = toupper((unsigned char)*p);
 		} while (*(++p));
 	}
 
@@ -882,7 +882,7 @@ int vsnprintf(char *buf, size_t size, const char *fmt, va_list args)
 		case FORMAT_TYPE_PTR:
 			str = pointer(fmt+1, str, end, va_arg(args, void *),
 				      spec);
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
@@ -1177,7 +1177,7 @@ do {									\
 		case FORMAT_TYPE_PTR:
 			save_arg(void *);
 			/* skip all alphanumeric pointer suffixes */
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;
 
@@ -1340,7 +1340,7 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const uint32_t *bin_buf
 
 		case FORMAT_TYPE_PTR:
 			str = pointer(fmt+1, str, end, get_arg(void *), spec);
-			while (isalnum(*fmt))
+			while (isalnum((unsigned char)*fmt))
 				fmt++;
 			break;