diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 25e3540f436e..52fe9757278f 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,16 @@
+2017-11-29  Michael Catanzaro  <mcatanzaro@igalia.com>
+
+        REGRESSION(r218064): [GTK] Broke entering fullscreen mode in debug builds
+        https://bugs.webkit.org/show_bug.cgi?id=180120
+
+        Reviewed by Carlos Garcia Campos.
+
+        These assertions need to be swapped. Fixes /webkit2/WebKitWebView/fullscreen in debug mode.
+
+        * UIProcess/API/gtk/WebKitWebViewBase.cpp:
+        (webkitWebViewBaseEnterFullScreen):
+        (webkitWebViewBaseExitFullScreen):
+
 2017-12-02  Carlos Garcia Campos  <cgarcia@igalia.com>
 
         WebDriver: handle user prompts shown while executing scripts
diff --git a/Source/WebKit/UIProcess/API/gtk/WebKitWebViewBase.cpp b/Source/WebKit/UIProcess/API/gtk/WebKitWebViewBase.cpp
index dfa0e548d843..ef6b90e41a26 100644
--- a/Source/WebKit/UIProcess/API/gtk/WebKitWebViewBase.cpp
+++ b/Source/WebKit/UIProcess/API/gtk/WebKitWebViewBase.cpp
@@ -1306,7 +1306,7 @@ void webkitWebViewBaseEnterFullScreen(WebKitWebViewBase* webkitWebViewBase)
 {
 #if ENABLE(FULLSCREEN_API)
     WebKitWebViewBasePrivate* priv = webkitWebViewBase->priv;
-    ASSERT(priv->fullScreenModeActive);
+    ASSERT(!priv->fullScreenModeActive);
 
     WebFullScreenManagerProxy* fullScreenManagerProxy = priv->pageProxy->fullScreenManager();
     fullScreenManagerProxy->willEnterFullScreen();
@@ -1324,7 +1324,7 @@ void webkitWebViewBaseExitFullScreen(WebKitWebViewBase* webkitWebViewBase)
 {
 #if ENABLE(FULLSCREEN_API)
     WebKitWebViewBasePrivate* priv = webkitWebViewBase->priv;
-    ASSERT(!priv->fullScreenModeActive);
+    ASSERT(priv->fullScreenModeActive);
 
     WebFullScreenManagerProxy* fullScreenManagerProxy = priv->pageProxy->fullScreenManager();
     fullScreenManagerProxy->willExitFullScreen();