diff --git a/dtc-lexer.l b/dtc-lexer.l
index 369407a..0cd7e67 100644
--- a/dtc-lexer.l
+++ b/dtc-lexer.l
@@ -75,11 +75,11 @@ static bool pop_input_file(void);
 			char *line, *tmp, *fn;
 			/* skip text before line # */
 			line = yytext;
-			while (!isdigit(*line))
+			while (!isdigit((unsigned char)*line))
 				line++;
 			/* skip digits in line # */
 			tmp = line;
-			while (!isspace(*tmp))
+			while (!isspace((unsigned char)*tmp))
 				tmp++;
 			/* "NULL"-terminate line # */
 			*tmp = '\0';
diff --git a/treesource.c b/treesource.c
index ffebb77..bf7a626 100644
--- a/treesource.c
+++ b/treesource.c
@@ -56,7 +56,7 @@ static void write_prefix(FILE *f, int level)
 
 static bool isstring(char c)
 {
-	return (isprint(c)
+	return (isprint((unsigned char)c)
 		|| (c == '\0')
 		|| strchr("\a\b\t\n\v\f\r", c));
 }
@@ -119,7 +119,7 @@ static void write_propval_string(FILE *f, struct data val)
 			fprintf(f, "\"");
 			break;
 		default:
-			if (isprint(c))
+			if (isprint((unsigned char)c))
 				fprintf(f, "%c", c);
 			else
 				fprintf(f, "\\x%02hhx", c);
diff --git a/util.c b/util.c
index 2347af9..330b594 100644
--- a/util.c
+++ b/util.c
@@ -87,7 +87,7 @@ bool util_is_printable_string(const void *data, int len)
 
 	while (s < se) {
 		ss = s;
-		while (s < se && *s && isprint(*s))
+		while (s < se && *s && isprint((unsigned char)*s))
 			s++;
 
 		/* not zero, or not done yet */