diff --git a/configure.ac b/configure.ac
index 05e5f64..f2773b1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -14,6 +14,9 @@ LT_INIT
 AC_CONFIG_MACRO_DIR([m4])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+dnl Check that compiler understands inline
+AC_C_INLINE
+
 #
 # Gettext
 #
diff --git a/src/psl.c b/src/psl.c
index 4bc6e53..6fe0633 100644
--- a/src/psl.c
+++ b/src/psl.c
@@ -497,6 +497,11 @@ const char *psl_registrable_domain(const psl_ctx_t *psl, const char *domain)
 	return regdom;
 }
 
+static inline int _isspace_ascii(const char c)
+{
+	return c == ' ' || c == '\t' || c == '\r' || c == '\n';
+}
+
 static int _str_is_ascii(const char *s)
 {
 	while (*s && *((unsigned char *)s) < 128) s++;
@@ -680,14 +685,14 @@ psl_ctx_t *psl_load_fp(FILE *fp)
 	psl->suffix_exceptions = _vector_alloc(64, _suffix_compare_array);
 
 	while ((linep = fgets(buf, sizeof(buf), fp))) {
-		while (isspace(*linep)) linep++; /* ignore leading whitespace */
+		while (_isspace_ascii(*linep)) linep++; /* ignore leading whitespace */
 		if (!*linep) continue; /* skip empty lines */
 
 		if (*linep == '/' && linep[1] == '/')
 			continue; /* skip comments */
 
 		/* parse suffix rule */
-		for (p = linep; *linep && !isspace(*linep);) linep++;
+		for (p = linep; *linep && !_isspace_ascii(*linep);) linep++;
 		*linep = 0;
 
 		if (*p == '!') {
diff --git a/tests/test-is-public-all.c b/tests/test-is-public-all.c
index 743004c..eb95e0e 100644
--- a/tests/test-is-public-all.c
+++ b/tests/test-is-public-all.c
@@ -46,6 +46,11 @@ static int
 	ok,
 	failed;
 
+static inline int _isspace_ascii(const char c)
+{
+	return c == ' ' || c == '\t' || c == '\r' || c == '\n';
+}
+
 static void test_psl(void)
 {
 	FILE *fp;
@@ -59,14 +64,14 @@ static void test_psl(void)
 
 	if ((fp = fopen(PSL_FILE, "r"))) {
 		while ((linep = fgets(buf, sizeof(buf), fp))) {
-			while (isspace(*linep)) linep++; /* ignore leading whitespace */
+			while (_isspace_ascii(*linep)) linep++; /* ignore leading whitespace */
 			if (!*linep) continue; /* skip empty lines */
 
 			if (*linep == '/' && linep[1] == '/')
 				continue; /* skip comments */
 
 			/* parse suffix rule */
-			for (p = linep; *linep && !isspace(*linep);) linep++;
+			for (p = linep; *linep && !_isspace_ascii(*linep);) linep++;
 			*linep = 0;
 
 			if (*p == '!') { /* an exception to a wildcard, e.g. !www.ck (wildcard is *.ck) */
diff --git a/tests/test-registrable-domain.c b/tests/test-registrable-domain.c
index 1d4b68d..cb6ae9b 100644
--- a/tests/test-registrable-domain.c
+++ b/tests/test-registrable-domain.c
@@ -35,7 +35,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <ctype.h>
 #ifdef HAVE_ALLOCA_H
 #	include <alloca.h>
 #endif