diff --git a/sk.c b/sk.c
index 84cf11e..b006c57 100644
--- a/sk.c
+++ b/sk.c
@@ -119,7 +119,7 @@ int sk_interface_phc(char *name, int *index)
 	close(fd);
 	*index = info.phc_index;
 
-	return 0;
+	return info.phc_index < 0 ? -1 : 0;
 #else
 	return -1;
 #endif
diff --git a/sk.h b/sk.h
index 5f1ee4b..b2787b2 100644
--- a/sk.h
+++ b/sk.h
@@ -33,7 +33,8 @@ int sk_interface_index(int fd, char *device);
 /**
  * Obtain the PHC device index of a network interface.
  * @param name	    The name of the interface
- * @return index    The phc index associated with this iface
+ * @return index    The non-negative phc index associated with this iface.
+ *                  On error a negative integer is returned.
  */
 int sk_interface_phc(char *name, int *index);