diff --git a/charset/localenc.c b/charset/localenc.c
index 4d81f50f..22088877 100644
--- a/charset/localenc.c
+++ b/charset/localenc.c
@@ -102,7 +102,7 @@ int charset_from_localenc(const char *name)
 	p = name;
 	q = localencs[i].name;
 	while (*p || *q) {
-	    if (tolower(*p) != tolower(*q))
+		if (tolower((unsigned char)*p) != tolower((unsigned char)*q))
 		break;
 	    p++; q++;
 	}
diff --git a/charset/mimeenc.c b/charset/mimeenc.c
index 437781f6..93ad89e7 100644
--- a/charset/mimeenc.c
+++ b/charset/mimeenc.c
@@ -202,7 +202,7 @@ int charset_from_mimeenc(const char *name)
 	p = name;
 	q = mimeencs[i].name;
 	while (*p || *q) {
-	    if (tolower(*p) != tolower(*q))
+		if (tolower((unsigned char)*p) != tolower((unsigned char)*q))
 		break;
 	    p++; q++;
 	}
diff --git a/charset/xenc.c b/charset/xenc.c
index b6532f29..11787978 100644
--- a/charset/xenc.c
+++ b/charset/xenc.c
@@ -81,7 +81,7 @@ int charset_from_xenc(const char *name)
 	p = name;
 	q = xencs[i].name;
 	while (*p || *q) {
-	    if (tolower(*p) != tolower(*q))
+		if (tolower((unsigned char)*p) != tolower((unsigned char)*q))
 		break;
 	    p++; q++;
 	}