diff --git a/src/Curl/Url.php b/src/Curl/Url.php
index 9c8df63..3c5b0f3 100644
--- a/src/Curl/Url.php
+++ b/src/Curl/Url.php
@@ -95,7 +95,7 @@ class Url
         if (!isset($b['path'])) {
             $b['path'] = '/';
         }
-        if (is_null($this->relativeUrl)) {
+        if ($this->relativeUrl === null) {
             return $this->unparseUrl($b);
         }
         $r = $this->parseUrl($this->relativeUrl);
diff --git a/tests/script.sh b/tests/script.sh
index 61b3786..ec41cc0 100755
--- a/tests/script.sh
+++ b/tests/script.sh
@@ -122,6 +122,13 @@ if [[ ! -z "${elses}" ]]; then
     ((errors++))
 fi
 
+# Prohibit use of "is_null" and suggest using the strict comparison operator.
+is_null=$(find . -type "f" -iname "*.php" ! -path "*/vendor/*" -exec grep --color=always --line-number -H -e "is_null" {} \;)
+if [[ ! -z "${is_null}" ]]; then
+    echo -e "${is_null}" | perl -pe 's/^(.*)$/is_null found in \1. Replace with strict comparison (e.g. "\$x === null")./'
+    ((errors++))
+fi
+
 # Detect coding standard violations.
 phpcs --version
 phpcs \