diff --git a/builtin/mailsplit.c b/builtin/mailsplit.c
index 30681681c1..c0d88f9751 100644
--- a/builtin/mailsplit.c
+++ b/builtin/mailsplit.c
@@ -232,7 +232,7 @@ static int split_mbox(const char *file, const char *dir, int allow_bare,
 
 	do {
 		peek = fgetc(f);
-	} while (isspace(peek));
+	} while (peek >= 0 && isspace(peek));
 	ungetc(peek, f);
 
 	if (strbuf_getwholeline(&buf, f, '\n')) {
diff --git a/mailinfo.c b/mailinfo.c
index a489d9d0fb..659136cc58 100644
--- a/mailinfo.c
+++ b/mailinfo.c
@@ -1094,7 +1094,7 @@ int mailinfo(struct mailinfo *mi, const char *msg, const char *patch)
 
 	do {
 		peek = fgetc(mi->input);
-	} while (isspace(peek));
+	} while (peek >= 0 && isspace(peek));
 	ungetc(peek, mi->input);
 
 	/* process the email header */