diff --git a/CHANGELOG.md b/CHANGELOG.md
index f4c4426b..2575b80e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,7 @@
 
 * `msgpack`: Now uses string encoding instead of float encoding for a whole number that is too large to fit in any of MessagePack's integer types.
 * `function/stdlib`: Type conversion functions (constructed with `MakeToFunc`) can now convert null values of unknown type into null values of the target type, rather than returning an unknown value in that case.
+* `json`: Will now correctly reject attempts to encode `cty.DynamicVal`, whereas before it would just produce an invalid JSON document without any error. (This is invalid because JSON encoding cannot support unknown values at all; `cty.DynamicVal` is a special case of unknown value where even the _type_ isn't known.)
 
 # 1.14.3 (February 29, 2024)
 
diff --git a/cty/json/marshal.go b/cty/json/marshal.go
index 7a14ce81..07d9f331 100644
--- a/cty/json/marshal.go
+++ b/cty/json/marshal.go
@@ -12,6 +12,9 @@ func marshal(val cty.Value, t cty.Type, path cty.Path, b *bytes.Buffer) error {
 	if val.IsMarked() {
 		return path.NewErrorf("value has marks, so it cannot be serialized as JSON")
 	}
+	if !val.IsKnown() {
+		return path.NewErrorf("value is not known")
+	}
 
 	// If we're going to decode as DynamicPseudoType then we need to save
 	// dynamic type information to recover the real type.
@@ -24,10 +27,6 @@ func marshal(val cty.Value, t cty.Type, path cty.Path, b *bytes.Buffer) error {
 		return nil
 	}
 
-	if !val.IsKnown() {
-		return path.NewErrorf("value is not known")
-	}
-
 	// The caller should've guaranteed that the given val is conformant with
 	// the given type t, so we'll proceed under that assumption here.
 
@@ -185,7 +184,10 @@ func marshalDynamic(val cty.Value, path cty.Path, b *bytes.Buffer) error {
 		return path.NewErrorf("failed to serialize type: %s", err)
 	}
 	b.WriteString(`{"value":`)
-	marshal(val, val.Type(), path, b)
+	err = marshal(val, val.Type(), path, b)
+	if err != nil {
+		return path.NewErrorf("failed to serialize value: %s", err)
+	}
 	b.WriteString(`,"type":`)
 	b.Write(typeJSON)
 	b.WriteRune('}')