diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index c52e3ed..b4b9385 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,30 @@
+2020-04-01  Wenson Hsieh  <wenson_hsieh@apple.com>
+
+        Make WebPasteboardProxy::didModifyContentsOfPasteboard robust when pasteboardName is null
+        https://bugs.webkit.org/show_bug.cgi?id=209848
+        <rdar://problem/61121810>
+
+        Reviewed by Megan Gardner and David Kilzer.
+
+        Add more IPC message checks in WebPasteboardProxy; see below for more detail.
+
+        * UIProcess/Cocoa/WebPasteboardProxyCocoa.mm:
+
+        Rename what is currently MESSAGE_CHECK to MESSAGE_CHECK_COMPLETION, and introduce two more message check macros:
+        MESSAGE_CHECK_WITH_RETURN_VALUE, which supports a return value, and MESSAGE_CHECK, which returns with no value.
+
+        (WebKit::WebPasteboardProxy::canAccessPasteboardData const):
+
+        Replace the early returns when pasteboardName is empty or when the web process for the given connection is null
+        with `MESSAGE_CHECK`s. When the web content process is well-behaved, these early returns should never be hit.
+
+        (WebKit::WebPasteboardProxy::didModifyContentsOfPasteboard):
+
+        Similarly, replace this early return with a message check, and additionally `MESSAGE_CHECK` when the pasteboard
+        name is empty. This addresses the main issue caught by this radar.
+
+        (WebKit::WebPasteboardProxy::setPasteboardBufferForType):
+
 2020-04-01  Victor M. Jaquez <vjaquez@igalia.com>
 
         Bump libwebrtc to M82
diff --git a/Source/WebKit/UIProcess/Cocoa/WebPasteboardProxyCocoa.mm b/Source/WebKit/UIProcess/Cocoa/WebPasteboardProxyCocoa.mm
index 4e3aa88..d5eedef 100644
--- a/Source/WebKit/UIProcess/Cocoa/WebPasteboardProxyCocoa.mm
+++ b/Source/WebKit/UIProcess/Cocoa/WebPasteboardProxyCocoa.mm
@@ -38,7 +38,9 @@
 #import <WebCore/SharedBuffer.h>
 #import <wtf/URL.h>
 
-#define MESSAGE_CHECK(assertion, completion) MESSAGE_CHECK_COMPLETION_BASE(assertion, (&connection), completion)
+#define MESSAGE_CHECK(assertion) MESSAGE_CHECK_BASE(assertion, (&connection))
+#define MESSAGE_CHECK_WITH_RETURN_VALUE(assertion, returnValue) MESSAGE_CHECK_WITH_RETURN_VALUE_BASE(assertion, (&connection), returnValue)
+#define MESSAGE_CHECK_COMPLETION(assertion, completion) MESSAGE_CHECK_COMPLETION_BASE(assertion, (&connection), completion)
 
 namespace WebKit {
 using namespace WebCore;
@@ -73,14 +75,10 @@
 
 bool WebPasteboardProxy::canAccessPasteboardData(IPC::Connection& connection, const String& pasteboardName) const
 {
-    if (pasteboardName.isEmpty()) {
-        ASSERT_NOT_REACHED();
-        return false;
-    }
+    MESSAGE_CHECK_WITH_RETURN_VALUE(!pasteboardName.isEmpty(), false);
 
     auto* process = webProcessProxyForConnection(connection);
-    if (!process)
-        return false;
+    MESSAGE_CHECK_WITH_RETURN_VALUE(process, false);
 
     for (auto* page : process->pages()) {
         auto& preferences = page->preferences();
@@ -106,9 +104,10 @@
 
 void WebPasteboardProxy::didModifyContentsOfPasteboard(IPC::Connection& connection, const String& pasteboardName, int64_t previousChangeCount, int64_t newChangeCount)
 {
+    MESSAGE_CHECK(!pasteboardName.isEmpty());
+
     auto* process = webProcessProxyForConnection(connection);
-    if (!process)
-        return;
+    MESSAGE_CHECK(process);
 
     auto changeCountAndProcesses = m_pasteboardNameToChangeCountAndProcessesMap.find(pasteboardName);
     if (changeCountAndProcesses != m_pasteboardNameToChangeCountAndProcessesMap.end() && previousChangeCount == changeCountAndProcesses->value.first) {
@@ -300,7 +299,7 @@
     }
 
     // SharedMemory::Handle::size() is rounded up to the nearest page.
-    MESSAGE_CHECK(size <= handle.size(), completionHandler(0));
+    MESSAGE_CHECK_COMPLETION(size <= handle.size(), completionHandler(0));
 
     RefPtr<SharedMemory> sharedMemoryBuffer = SharedMemory::map(handle, SharedMemory::Protection::ReadOnly);
     if (!sharedMemoryBuffer)