diff --git a/include/swift/AST/DiagnosticsCommon.def b/include/swift/AST/DiagnosticsCommon.def
index 0047fd8a2c1..65b7728ebd0 100644
--- a/include/swift/AST/DiagnosticsCommon.def
+++ b/include/swift/AST/DiagnosticsCommon.def
@@ -94,9 +94,11 @@ ERROR(function_type_no_parens,none,
       "single argument function types require parentheses", ())
 
 // FIXME: Used by swift-api-digester. Don't want to set up a separate diagnostics
-// file just for one error.
+// file just for two errors.
 ERROR(sdk_node_unrecognized_key,none,
       "unrecognized key '%0' in SDK node", (StringRef))
+ERROR(sdk_node_unrecognized_node_kind,none,
+      "unrecognized SDK node kind '%0'", (StringRef))
 
 //------------------------------------------------------------------------------
 // MARK: Circular reference diagnostics
diff --git a/include/swift/IDE/APIDigesterData.h b/include/swift/IDE/APIDigesterData.h
index 2e0cc6cdce1..9c0d6f07fa8 100644
--- a/include/swift/IDE/APIDigesterData.h
+++ b/include/swift/IDE/APIDigesterData.h
@@ -33,7 +33,7 @@ enum class SDKNodeKind: uint8_t {
 #include "DigesterEnums.def"
 };
 
-SDKNodeKind parseSDKNodeKind(StringRef Content);
+Optional<SDKNodeKind> parseSDKNodeKind(StringRef Content);
 
 enum class NodeAnnotation: uint8_t{
 #define NODE_ANNOTATION(NAME) NAME,
diff --git a/lib/IDE/APIDigesterData.cpp b/lib/IDE/APIDigesterData.cpp
index 9824a827131..e849672af56 100644
--- a/lib/IDE/APIDigesterData.cpp
+++ b/lib/IDE/APIDigesterData.cpp
@@ -39,10 +39,11 @@ operator<<(raw_ostream &Out, const NodeAnnotation Value) {
   llvm_unreachable("Undefined SDK node kind.");
 }
 
-SDKNodeKind swift::ide::api::parseSDKNodeKind(StringRef Content) {
-  return llvm::StringSwitch<SDKNodeKind>(Content)
+Optional<SDKNodeKind> swift::ide::api::parseSDKNodeKind(StringRef Content) {
+  return llvm::StringSwitch<Optional<SDKNodeKind>>(Content)
 #define NODE_KIND(NAME, VALUE) .Case(#VALUE, SDKNodeKind::NAME)
 #include "swift/IDE/DigesterEnums.def"
+    .Default(None)
   ;
 }
 
@@ -326,7 +327,7 @@ serializeDiffItem(llvm::BumpPtrAllocator &Alloc,
   switch (parseDiffItemKind(DiffItemKind)) {
   case APIDiffItemKind::ADK_CommonDiffItem: {
     return new (Alloc.Allocate<CommonDiffItem>())
-      CommonDiffItem(parseSDKNodeKind(NodeKind),
+      CommonDiffItem(*parseSDKNodeKind(NodeKind),
                      parseSDKNodeAnnotation(NodeAnnotation), ChildIndex,
                      LeftUsr, RightUsr, LeftComment, RightComment, ModuleName);
   }
diff --git a/test/api-digester/diagnostics.json b/test/api-digester/diagnostics.json
index 2c1a9e1223b..81ef51ea20e 100644
--- a/test/api-digester/diagnostics.json
+++ b/test/api-digester/diagnostics.json
@@ -2,6 +2,8 @@
   "kind": "Root",
   "name": "TopLevel",
   "printedName": "TopLevel",
-  "children": [],
-  "badKey": ["foo", "bar", "baz"]
+  "badKey": ["foo", "bar", "baz"],
+  "children": [
+    { "kind": "Zyzyx" }
+  ]
 }
diff --git a/test/api-digester/diagnostics.swift b/test/api-digester/diagnostics.swift
index c8b60f138fc..2bfaede6607 100644
--- a/test/api-digester/diagnostics.swift
+++ b/test/api-digester/diagnostics.swift
@@ -1,7 +1,8 @@
 // REQUIRES: OS=macosx
 // RUN: not %api-digester -deserialize-sdk -input-paths %S/diagnostics.json -o - 2>&1 | %FileCheck %s
 
-// CHECK: diagnostics.json:6:3: error: unrecognized key 'badKey' in SDK node
+// CHECK: diagnostics.json:5:3: error: unrecognized key 'badKey' in SDK node
+// CHECK: diagnostics.json:7:15: error: unrecognized SDK node kind 'Zyzyx'
 
 // Make sure we don't try to output a result:
 // CHECK-NOT: "kind": "Root",
diff --git a/tools/swift-api-digester/swift-api-digester.cpp b/tools/swift-api-digester/swift-api-digester.cpp
index 27361d0e123..61b6993038b 100644
--- a/tools/swift-api-digester/swift-api-digester.cpp
+++ b/tools/swift-api-digester/swift-api-digester.cpp
@@ -1057,7 +1057,14 @@ SDKNode* SDKNode::constructSDKNode(SDKContext &Ctx,
     if (auto keyKind = parseKeyKind(keyString)) {
       switch(*keyKind) {
       case KeyKind::KK_kind:
-        Kind = parseSDKNodeKind(GetScalarString(Pair.getValue()));
+        if (auto parsedKind = parseSDKNodeKind(GetScalarString(Pair.getValue()))) {
+          Kind = *parsedKind;
+        } else {
+          auto range = convertRange(Pair.getValue()->getSourceRange());
+          Ctx.getDiags().diagnose(range.Start, diag::sdk_node_unrecognized_node_kind,
+                                  GetScalarString(Pair.getValue()))
+            .highlight(range);
+        }
         break;
       case KeyKind::KK_name:
         Info.Name = GetScalarString(Pair.getValue());