diff --git a/lib/serial/README.md b/lib/serial/README.md
index 7e8d4a7..065a491 100644
--- a/lib/serial/README.md
+++ b/lib/serial/README.md
@@ -67,7 +67,7 @@ print(dumps({'b': 2, 'a': 1}))
 
 Reconstruct the value from a `dumps` string, interpreting the type tags. The result is a fresh, **unfrozen** value (the same as `json.decode`), so scripts can read or mutate it.
 
-**Errors on**: invalid JSON (`serial.loads: …`), an unknown type tag (`unknown type tag "…"`), or a malformed envelope payload (`invalid bytes payload`, `invalid bigint payload`, `invalid time payload`, `invalid mapkv entry`, `invalid object payload`). A `set` with an unhashable element or a `mapkv` with an unhashable key errors (`unhashable`). Bare JSON numbers decode without a tag: an integer (any precision) to `int`, a number with `.`/`e`/`E` to `float`.
+**Errors on**: invalid JSON (`serial.loads: …`), trailing content after the value (`unexpected trailing data after JSON value` — a second JSON document or garbage; trailing whitespace is fine), an unknown type tag (`unknown type tag "…"`), or a malformed envelope payload whose `v` has the wrong shape (`invalid bytes payload`, `invalid bigint payload`, `invalid time payload`, `invalid tuple payload`, `invalid set payload`, `invalid mapkv payload`/`invalid mapkv entry`, `invalid object payload`). A `set` with an unhashable element or a `mapkv` with an unhashable key errors (`unhashable`). Bare JSON numbers decode without a tag: an integer (any precision) to `int`, a number with `.`/`e`/`E` to `float`.
 
 ```python
 load('serial', 'loads')
diff --git a/lib/serial/serial.go b/lib/serial/serial.go
index 0059907..1011760 100644
--- a/lib/serial/serial.go
+++ b/lib/serial/serial.go
@@ -124,6 +124,13 @@ func generateLoads(try bool) func(*starlark.Thread, *starlark.Builtin, starlark.
 		if err := dec.Decode(&raw); err != nil {
 			return failResult(try, err, fn, true)
 		}
+		// loads round-trips a single value; reject a second JSON value or
+		// trailing garbage rather than silently dropping it. dec.More() is
+		// true only for a further non-whitespace token, so trailing
+		// whitespace/newlines still pass.
+		if dec.More() {
+			return failResult(try, fmt.Errorf("unexpected trailing data after JSON value"), fn, true)
+		}
 		val, err := decode(raw)
 		if err != nil {
 			return failResult(try, err, fn, true)
@@ -399,7 +406,10 @@ func decodeObject(m map[string]interface{}) (starlark.Value, error) {
 }
 
 func decodeBytes(raw interface{}) (starlark.Value, error) {
-	s, _ := raw.(string)
+	s, ok := raw.(string)
+	if !ok {
+		return nil, fmt.Errorf("invalid bytes payload: expected a base64 string")
+	}
 	b, err := base64.StdEncoding.DecodeString(s)
 	if err != nil {
 		return nil, fmt.Errorf("invalid bytes payload: %w", err)
@@ -408,16 +418,22 @@ func decodeBytes(raw interface{}) (starlark.Value, error) {
 }
 
 func decodeBigint(raw interface{}) (starlark.Value, error) {
-	s, _ := raw.(string)
-	bi, ok := new(big.Int).SetString(s, 10)
+	s, ok := raw.(string)
 	if !ok {
+		return nil, fmt.Errorf("invalid bigint payload: expected a decimal string")
+	}
+	bi, valid := new(big.Int).SetString(s, 10)
+	if !valid {
 		return nil, fmt.Errorf("invalid bigint payload %q", s)
 	}
 	return starlark.MakeBigInt(bi), nil
 }
 
 func decodeTuple(raw interface{}) (starlark.Value, error) {
-	arr, _ := raw.([]interface{})
+	arr, ok := raw.([]interface{})
+	if !ok {
+		return nil, fmt.Errorf("invalid tuple payload: expected an array")
+	}
 	elems := make([]starlark.Value, len(arr))
 	for i, e := range arr {
 		ev, err := decode(e)
@@ -430,7 +446,10 @@ func decodeTuple(raw interface{}) (starlark.Value, error) {
 }
 
 func decodeSet(raw interface{}) (starlark.Value, error) {
-	arr, _ := raw.([]interface{})
+	arr, ok := raw.([]interface{})
+	if !ok {
+		return nil, fmt.Errorf("invalid set payload: expected an array")
+	}
 	set := starlark.NewSet(len(arr))
 	for _, e := range arr {
 		ev, err := decode(e)
@@ -445,7 +464,10 @@ func decodeSet(raw interface{}) (starlark.Value, error) {
 }
 
 func decodeTime(raw interface{}) (starlark.Value, error) {
-	s, _ := raw.(string)
+	s, ok := raw.(string)
+	if !ok {
+		return nil, fmt.Errorf("invalid time payload: expected an RFC3339 string")
+	}
 	tm, err := time.Parse(time.RFC3339Nano, s)
 	if err != nil {
 		return nil, fmt.Errorf("invalid time payload %q: %w", s, err)
@@ -454,7 +476,10 @@ func decodeTime(raw interface{}) (starlark.Value, error) {
 }
 
 func decodeMapKV(raw interface{}) (starlark.Value, error) {
-	arr, _ := raw.([]interface{})
+	arr, ok := raw.([]interface{})
+	if !ok {
+		return nil, fmt.Errorf("invalid mapkv payload: expected an array of [key, value] pairs")
+	}
 	d := starlark.NewDict(len(arr))
 	for _, pr := range arr {
 		kvp, ok := pr.([]interface{})
diff --git a/lib/serial/serial_test.go b/lib/serial/serial_test.go
index 83b2eee..c866cd3 100644
--- a/lib/serial/serial_test.go
+++ b/lib/serial/serial_test.go
@@ -387,6 +387,36 @@ func TestLoadModule_Serial(t *testing.T) {
 				`),
 			wantErr: `unknown type tag`,
 		},
+		{
+			// a wrong-typed envelope payload (e.g. a number where an array or
+			// string is expected) used to be silently coerced to an empty
+			// value instead of erroring, violating the lossless-or-error
+			// contract. Each tag decoder must now reject the wrong shape.
+			name: `error: malformed envelope payloads reject the wrong type`,
+			script: itn.HereDoc(`
+				load('serial', 'loads')
+				assert.fails(lambda: loads('{"$t":"tuple","v":123}'), 'invalid tuple payload')
+				assert.fails(lambda: loads('{"$t":"set","v":123}'), 'invalid set payload')
+				assert.fails(lambda: loads('{"$t":"mapkv","v":123}'), 'invalid mapkv payload')
+				assert.fails(lambda: loads('{"$t":"bytes","v":123}'), 'invalid bytes payload')
+				assert.fails(lambda: loads('{"$t":"bigint","v":123}'), 'invalid bigint payload')
+				assert.fails(lambda: loads('{"$t":"time","v":123}'), 'invalid time payload')
+			`),
+		},
+		{
+			// loads round-trips a single value; a second JSON value or
+			// trailing garbage was silently dropped. Trailing whitespace is
+			// still fine.
+			name: `error: trailing content after the JSON value is rejected`,
+			script: itn.HereDoc(`
+				load('serial', 'loads')
+				assert.eq(loads('1'), 1)
+				assert.eq(loads('  [1, 2]  \n'), [1, 2])
+				assert.fails(lambda: loads('1 2'), 'trailing data')
+				assert.fails(lambda: loads('{"a":1}{"b":2}'), 'trailing data')
+				assert.fails(lambda: loads('[1,2] junk'), 'trailing data')
+			`),
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {