diff --git a/lib/openpam_readline.c b/lib/openpam_readline.c
index 351966f..9a6fdf6 100644
--- a/lib/openpam_readline.c
+++ b/lib/openpam_readline.c
@@ -84,7 +84,7 @@ openpam_readline(FILE *f, int *lineno, size_t *lenp)
 		/* eof */
 		if (ch == EOF) {
 			/* remove trailing whitespace */
-			while (len > 0 && isspace((int)line[len - 1]))
+			while (len > 0 && isspace((unsigned char)line[len - 1]))
 				--len;
 			line[len] = '\0';
 			if (len == 0)
@@ -97,7 +97,7 @@ openpam_readline(FILE *f, int *lineno, size_t *lenp)
 				++*lineno;
 
 			/* remove trailing whitespace */
-			while (len > 0 && isspace((int)line[len - 1]))
+			while (len > 0 && isspace((unsigned char)line[len - 1]))
 				--len;
 			line[len] = '\0';
 			/* skip blank lines */