diff --git a/lib/isc/include/isc/stdio.h b/lib/isc/include/isc/stdio.h
index 1f44b5a..a3625f9 100644
--- a/lib/isc/include/isc/stdio.h
+++ b/lib/isc/include/isc/stdio.h
@@ -69,6 +69,9 @@ isc_stdio_sync(FILE *f);
  * direct counterpart in the stdio library.
  */
 
+isc_result_t
+isc_stdio_fgetc(FILE *f, int *ret);
+
 ISC_LANG_ENDDECLS
 
 #endif /* ISC_STDIO_H */
diff --git a/lib/isc/lex.c b/lib/isc/lex.c
index ca5fe6d..b0152eb 100644
--- a/lib/isc/lex.c
+++ b/lib/isc/lex.c
@@ -433,15 +433,11 @@ isc_lex_gettoken(isc_lex_t *lex, unsigned int options, isc_token_t *tokenp) {
 			if (source->is_file) {
 				stream = source->input;
 
-#if defined(HAVE_FLOCKFILE) && defined(HAVE_GETC_UNLOCKED)
-				c = getc_unlocked(stream);
-#else
-				c = getc(stream);
-#endif
-				if (c == EOF) {
-					if (ferror(stream)) {
-						source->result = ISC_R_IOERROR;
-						result = source->result;
+				result = isc_stdio_fgetc(stream, &c);
+
+				if (result != ISC_R_SUCCESS) {
+					if (result != ISC_R_EOF) {
+						source->result = result;
 						goto done;
 					}
 					source->at_eof = true;
diff --git a/lib/isc/unix/errno2result.c b/lib/isc/unix/errno2result.c
index d72d56f..4287ff3 100644
--- a/lib/isc/unix/errno2result.c
+++ b/lib/isc/unix/errno2result.c
@@ -42,6 +42,7 @@ isc___errno2result(int posixerrno, bool dolog,
 	case EINVAL:		/* XXX sometimes this is not for files */
 	case ENAMETOOLONG:
 	case EBADF:
+	case EISDIR:
 		return (ISC_R_INVALIDFILE);
 	case ENOENT:
 		return (ISC_R_FILENOTFOUND);
diff --git a/lib/isc/unix/stdio.c b/lib/isc/unix/stdio.c
index d2b1dda..79630b2 100644
--- a/lib/isc/unix/stdio.c
+++ b/lib/isc/unix/stdio.c
@@ -141,3 +141,22 @@ isc_stdio_sync(FILE *f) {
 		return (isc__errno2result(errno));
 }
 
+isc_result_t
+isc_stdio_fgetc(FILE *f, int *ret) {
+	int r;
+	isc_result_t result = ISC_R_SUCCESS;
+
+#if defined(HAVE_FLOCKFILE) && defined(HAVE_GETCUNLOCKED)
+	r = fgetc_unlocked(f);
+#else
+	r = fgetc(f);
+#endif
+
+	if (r == EOF)
+		result = ferror(f) ? isc__errno2result(errno) : ISC_R_EOF;
+
+	*ret = r;
+
+	return result;
+}
+