diff --git a/lib/class-wp-rest-oauth1.php b/lib/class-wp-rest-oauth1.php
index 268001f..edc2538 100644
--- a/lib/class-wp-rest-oauth1.php
+++ b/lib/class-wp-rest-oauth1.php
@@ -77,9 +77,11 @@ public function get_authorization_header() {
 			$headers = getallheaders();
 
 			// Check for the authoization header case-insensitively
-			foreach ( $headers as $key => $value ) {
-				if ( strtolower( $key ) === 'authorization' ) {
-					return $value;
+			if ( is_array( $headers ) ) {
+				foreach ( $headers as $key => $value ) {
+					if ( strtolower( $key ) === 'authorization' ) {
+						return $value;
+					}
 				}
 			}
 		}