diff --git a/src/Iri.php b/src/Iri.php
index f5f451962..ac5ecbeb5 100644
--- a/src/Iri.php
+++ b/src/Iri.php
@@ -216,7 +216,7 @@ public function __get($name) {
 			$return = null;
 		}
 
-		if ($return === null && isset($this->normalization[$this->scheme][$name])) {
+		if ($return === null && isset($this->scheme, $this->normalization[$this->scheme][$name])) {
 			return $this->normalization[$this->scheme][$name];
 		}
 		else {
@@ -671,27 +671,29 @@ protected function remove_iunreserved_percent_encoded($regex_match) {
 	}
 
 	protected function scheme_normalization() {
-		if (isset($this->normalization[$this->scheme]['iuserinfo']) && $this->iuserinfo === $this->normalization[$this->scheme]['iuserinfo']) {
-			$this->iuserinfo = null;
-		}
-		if (isset($this->normalization[$this->scheme]['ihost']) && $this->ihost === $this->normalization[$this->scheme]['ihost']) {
-			$this->ihost = null;
-		}
-		if (isset($this->normalization[$this->scheme]['port']) && $this->port === $this->normalization[$this->scheme]['port']) {
-			$this->port = null;
-		}
-		if (isset($this->normalization[$this->scheme]['ipath']) && $this->ipath === $this->normalization[$this->scheme]['ipath']) {
-			$this->ipath = '';
+		if (isset($this->scheme, $this->normalization[$this->scheme])) {
+			if (isset($this->normalization[$this->scheme]['iuserinfo']) && $this->iuserinfo === $this->normalization[$this->scheme]['iuserinfo']) {
+				$this->iuserinfo = null;
+			}
+			if (isset($this->normalization[$this->scheme]['ihost']) && $this->ihost === $this->normalization[$this->scheme]['ihost']) {
+				$this->ihost = null;
+			}
+			if (isset($this->normalization[$this->scheme]['port']) && $this->port === $this->normalization[$this->scheme]['port']) {
+				$this->port = null;
+			}
+			if (isset($this->normalization[$this->scheme]['ipath']) && $this->ipath === $this->normalization[$this->scheme]['ipath']) {
+				$this->ipath = '';
+			}
+			if (isset($this->normalization[$this->scheme]['iquery']) && $this->iquery === $this->normalization[$this->scheme]['iquery']) {
+				$this->iquery = null;
+			}
+			if (isset($this->normalization[$this->scheme]['ifragment']) && $this->ifragment === $this->normalization[$this->scheme]['ifragment']) {
+				$this->ifragment = null;
+			}
 		}
 		if (isset($this->ihost) && empty($this->ipath)) {
 			$this->ipath = '/';
 		}
-		if (isset($this->normalization[$this->scheme]['iquery']) && $this->iquery === $this->normalization[$this->scheme]['iquery']) {
-			$this->iquery = null;
-		}
-		if (isset($this->normalization[$this->scheme]['ifragment']) && $this->ifragment === $this->normalization[$this->scheme]['ifragment']) {
-			$this->ifragment = null;
-		}
 	}
 
 	/**
diff --git a/src/Response/Headers.php b/src/Response/Headers.php
index 1500c6754..0b423e4f3 100644
--- a/src/Response/Headers.php
+++ b/src/Response/Headers.php
@@ -37,7 +37,7 @@ public function offsetGet($offset) {
 			$offset = strtolower($offset);
 		}
 
-		if (!isset($this->data[$offset])) {
+		if (!isset($offset, $this->data[$offset])) {
 			return null;
 		}
 
diff --git a/src/Utility/CaseInsensitiveDictionary.php b/src/Utility/CaseInsensitiveDictionary.php
index 9abbb7912..336804750 100644
--- a/src/Utility/CaseInsensitiveDictionary.php
+++ b/src/Utility/CaseInsensitiveDictionary.php
@@ -51,6 +51,10 @@ public function offsetExists($offset) {
 			$offset = strtolower($offset);
 		}
 
+		if ($offset === null) {
+			$offset = '';
+		}
+
 		return isset($this->data[$offset]);
 	}
 
@@ -66,6 +70,10 @@ public function offsetGet($offset) {
 			$offset = strtolower($offset);
 		}
 
+		if ($offset === null) {
+			$offset = '';
+		}
+
 		if (!isset($this->data[$offset])) {
 			return null;
 		}
@@ -91,6 +99,10 @@ public function offsetSet($offset, $value) {
 			$offset = strtolower($offset);
 		}
 
+		if ($offset === null) {
+			$offset = '';
+		}
+
 		$this->data[$offset] = $value;
 	}
 
@@ -105,6 +117,10 @@ public function offsetUnset($offset) {
 			$offset = strtolower($offset);
 		}
 
+		if ($offset === null) {
+			$offset = '';
+		}
+
 		unset($this->data[$offset]);
 	}