diff --git a/pkl-core/src/main/java/org/pkl/core/stdlib/json/ParserNodes.java b/pkl-core/src/main/java/org/pkl/core/stdlib/json/ParserNodes.java
index 1321674dc..c5c90c8ed 100644
--- a/pkl-core/src/main/java/org/pkl/core/stdlib/json/ParserNodes.java
+++ b/pkl-core/src/main/java/org/pkl/core/stdlib/json/ParserNodes.java
@@ -27,6 +27,7 @@
 import org.pkl.core.stdlib.ExternalMethod1Node;
 import org.pkl.core.stdlib.PklConverter;
 import org.pkl.core.util.EconomicMaps;
+import org.pkl.core.util.ErrorMessages;
 import org.pkl.core.util.Nullable;
 import org.pkl.core.util.json.JsonHandler;
 import org.pkl.core.util.json.JsonParser;
@@ -172,7 +173,13 @@ public void endObject(@Nullable EconomicMap<Object, ObjectMember> members) {
 
     @Override
     public void startObjectValue(@Nullable EconomicMap<Object, ObjectMember> members, String name) {
-      currPath.push(Identifier.get(name));
+      var identifier = Identifier.get(name);
+      if (!useMapping && identifier == Identifier.DEFAULT) {
+        // https://github.com/apple/pkl/issues/561
+        throw new ParseException(
+            ErrorMessages.create("jsonParseErrorDynamicPropertyDefault"), getLocation());
+      }
+      currPath.push(identifier);
     }
 
     @Override
diff --git a/pkl-core/src/main/java/org/pkl/core/stdlib/yaml/ParserNodes.java b/pkl-core/src/main/java/org/pkl/core/stdlib/yaml/ParserNodes.java
index fbe28890b..7dedac4cc 100644
--- a/pkl-core/src/main/java/org/pkl/core/stdlib/yaml/ParserNodes.java
+++ b/pkl-core/src/main/java/org/pkl/core/stdlib/yaml/ParserNodes.java
@@ -31,6 +31,8 @@
 import org.pkl.core.stdlib.ExternalMethod1Node;
 import org.pkl.core.stdlib.PklConverter;
 import org.pkl.core.util.EconomicMaps;
+import org.pkl.core.util.ErrorMessages;
+import org.pkl.core.util.yaml.ParseException;
 import org.pkl.core.util.yaml.snake.YamlUtils;
 import org.snakeyaml.engine.v2.api.ConstructNode;
 import org.snakeyaml.engine.v2.api.Load;
@@ -115,6 +117,8 @@ private VmList doParseAll(VmTyped self, String text, String uri) {
               .build();
         }
         throw exceptionBuilder().evalError("yamlParseError").withHint(e.getMessage()).build();
+      } catch (ParseException e) {
+        throw exceptionBuilder().evalError("yamlParseError").withHint(e.getMessage()).build();
       }
 
       return builder.build();
@@ -469,6 +473,13 @@ private void addMembers(MappingNode node, VmObject object) {
           var memberName =
               convertedKey instanceof String string && !useMapping ? Identifier.get(string) : null;
 
+          // https://github.com/apple/pkl/issues/561
+          if (memberName == Identifier.DEFAULT) {
+            throw new ParseException(
+                ErrorMessages.create("yamlParseErrorDynamicPropertyDefault"),
+                keyNode.getStartMark().isEmpty() ? null : keyNode.getStartMark().get());
+          }
+
           var member =
               new ObjectMember(
                   sourceSection,
diff --git a/pkl-core/src/main/java/org/pkl/core/util/yaml/ParseException.java b/pkl-core/src/main/java/org/pkl/core/util/yaml/ParseException.java
new file mode 100644
index 000000000..aa04b6219
--- /dev/null
+++ b/pkl-core/src/main/java/org/pkl/core/util/yaml/ParseException.java
@@ -0,0 +1,38 @@
+/*
+ * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.util.yaml;
+
+import org.pkl.core.util.Nullable;
+import org.snakeyaml.engine.v2.exceptions.Mark;
+
+/** An unchecked exception to indicate that an input does not qualify as valid YAML. */
+public final class ParseException extends RuntimeException {
+  private final @Nullable Mark location;
+
+  public ParseException(String message, @Nullable Mark location) {
+    super(location == null ? message : message + location);
+    this.location = location;
+  }
+
+  /**
+   * Returns the location at which the error occurred.
+   *
+   * @return the error location
+   */
+  public @Nullable Mark getLocation() {
+    return location;
+  }
+}
diff --git a/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties b/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
index 193045e2d..d390a46c9 100644
--- a/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
+++ b/pkl-core/src/main/resources/org/pkl/core/errorMessages.properties
@@ -796,6 +796,11 @@ Converter path `{0}` has invalid syntax.
 jsonParseError=\
 Error parsing JSON document.
 
+jsonParseErrorDynamicPropertyDefault=\
+Cannot parse an object with key `"default"` into a `Dynamic`.\
+\n\
+Try parsing into a `Mapping` instead, by setting `useMapping = true` in the `pkl.json#Parser`.
+
 yamlParseError=\
 Error parsing YAML document.
 
@@ -804,6 +809,11 @@ Error parsing YAML document: The number of aliases for collection nodes exceeds
 \n\
 To increase the allowed maximum, set `YamlRenderer.maxCollectionAliases`.
 
+yamlParseErrorDynamicPropertyDefault=\
+Cannot parse an object with key `default` into a `Dynamic`.\
+\n\
+Try parsing into a `Mapping` instead, by setting `useMapping = true` in the `pkl.yaml#Parser`.
+
 evaluationTimedOut=\
 Evaluation timed out after {0,number,#.##} second(s).
 
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/api/jsonParser1.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/api/jsonParser1.pkl
index b4c7fabfd..dbdb40863 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/input/api/jsonParser1.pkl
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/api/jsonParser1.pkl
@@ -66,3 +66,11 @@ res16 = parser.parse("""
 
 // invalid syntax
 res17 = test.catch(() -> parser.parse("!@#$%"))
+
+res18 = test.catch(() -> parser.parse("""
+  {"default": null}
+  """))
+
+res19 = (parser) { useMapping = true }.parse("""
+  {"default": null}
+  """)
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlParser2.pkl b/pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlParser2.pkl
index 043784188..fcf2bcf13 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlParser2.pkl
+++ b/pkl-core/src/test/files/LanguageSnippetTests/input/api/yamlParser2.pkl
@@ -91,3 +91,11 @@ res15 = parser.parseAll("""
   hobby: surfing
   ...
   """)
+
+res16 = test.catch(() -> parser.parse("""
+  {"default": null}
+  """))
+
+res17 = (parser) { useMapping = true }.parse("""
+  default: null
+  """)
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/api/jsonParser1.pcf b/pkl-core/src/test/files/LanguageSnippetTests/output/api/jsonParser1.pcf
index b51ba3d1c..0ac6d7813 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/output/api/jsonParser1.pcf
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/api/jsonParser1.pcf
@@ -48,3 +48,7 @@ res16 {
   }
 }
 res17 = "Error parsing JSON document."
+res18 = "Error parsing JSON document."
+res19 {
+  ["default"] = null
+}
diff --git a/pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlParser2.pcf b/pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlParser2.pcf
index 66336fde7..933efb6cd 100644
--- a/pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlParser2.pcf
+++ b/pkl-core/src/test/files/LanguageSnippetTests/output/api/yamlParser2.pcf
@@ -47,3 +47,7 @@ res15 = List(new {
 }, new {
   hobby = "surfing"
 })
+res16 = "Error parsing YAML document."
+res17 {
+  ["default"] = null
+}