diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index 99d1f8f5d0a9..fb3816e3c662 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,17 @@
+2017-08-22  Zan Dobersek  <zdobersek@igalia.com>
+
+        GLContext: zero-initialize the GLContext pointer in ThreadGlobalGLContext
+        https://bugs.webkit.org/show_bug.cgi?id=175819
+
+        Reviewed by Xabier Rodriguez-Calvar.
+
+        * platform/graphics/GLContext.cpp: The ThreadGlobalGLContext object is
+        allocated on heap, so the embedded GLContext pointer can contain a
+        non-null value that can cause problems when e.g. checking for a current
+        GLContext on some specific thread on which a GLContext hasn't yet been
+        made current. Zero-initializing this pointer will avoid false positives
+        that can occur in these circumstances.
+
 2017-08-21  Daniel Bates  <dabates@apple.com>
 
         Cleanup TextPainter
diff --git a/Source/WebCore/platform/graphics/GLContext.cpp b/Source/WebCore/platform/graphics/GLContext.cpp
index 33c97231017a..599dcc017278 100644
--- a/Source/WebCore/platform/graphics/GLContext.cpp
+++ b/Source/WebCore/platform/graphics/GLContext.cpp
@@ -50,7 +50,7 @@ class ThreadGlobalGLContext {
     GLContext* context() { return m_context; }
 
 private:
-    GLContext* m_context;
+    GLContext* m_context { nullptr };
 };
 
 ThreadSpecific<ThreadGlobalGLContext>* ThreadGlobalGLContext::staticGLContext;