diff --git a/src/zcbor_decode.c b/src/zcbor_decode.c
index df012cb8..1ced3e0b 100644
--- a/src/zcbor_decode.c
+++ b/src/zcbor_decode.c
@@ -1100,13 +1100,21 @@ bool zcbor_simple_decode(zcbor_state_t *state, uint8_t *result)
 	PRINT_FUNC();
 	INITIAL_CHECKS_WITH_TYPE(ZCBOR_MAJOR_TYPE_SIMPLE);
 
+	uint8_t additional = ZCBOR_ADDITIONAL(*state->payload);
+
 	/* Simple values must be 0-23 (additional is 0-23) or 24-255 (additional is 24).
 	 * Other additional values are not considered simple values. */
-	ZCBOR_ERR_IF(ZCBOR_ADDITIONAL(*state->payload) > 24, ZCBOR_ERR_WRONG_TYPE);
+	ZCBOR_ERR_IF(additional > ZCBOR_VALUE_IS_1_BYTE, ZCBOR_ERR_WRONG_TYPE);
 
 	if (!value_extract(state, result, sizeof(*result), NULL)) {
 		ZCBOR_FAIL();
 	}
+
+	/* Simple values less than 24 MUST be encoded in the additional information.
+	   Simple values 24 to 31 inclusive are unused. Ref: RFC8949 sec 3.3 */
+	if ((additional == ZCBOR_VALUE_IS_1_BYTE) && (*result < 32)) {
+		ERR_RESTORE(ZCBOR_ERR_INVALID_VALUE_ENCODING);
+	}
 	return true;
 }
 
diff --git a/src/zcbor_encode.c b/src/zcbor_encode.c
index 18207f56..1240fd9b 100644
--- a/src/zcbor_encode.c
+++ b/src/zcbor_encode.c
@@ -445,6 +445,10 @@ bool zcbor_list_map_end_force_encode(zcbor_state_t *state)
 
 bool zcbor_simple_encode(zcbor_state_t *state, uint8_t *input)
 {
+	/* Simple values 24 to 31 inclusive are unused. Ref: RFC8949 sec 3.3 */
+	if ((*input > ZCBOR_VALUE_IN_HEADER) && (*input < 32)) {
+		ZCBOR_ERR(ZCBOR_ERR_INVALID_VALUE_ENCODING);
+	}
 	if (!value_encode(state, ZCBOR_MAJOR_TYPE_SIMPLE, input, sizeof(*input))) {
 		zcbor_log("Error encoding %u (0x%p)\r\n", *input, input);
 		ZCBOR_FAIL();
@@ -455,7 +459,7 @@ bool zcbor_simple_encode(zcbor_state_t *state, uint8_t *input)
 
 bool zcbor_simple_put(zcbor_state_t *state, uint8_t input)
 {
-	return value_encode(state, ZCBOR_MAJOR_TYPE_SIMPLE, &input, sizeof(input));
+	return zcbor_simple_encode(state, &input);
 }
 
 
diff --git a/tests/unit/test1_unit_tests/src/main.c b/tests/unit/test1_unit_tests/src/main.c
index 106b4997..990a0a25 100644
--- a/tests/unit/test1_unit_tests/src/main.c
+++ b/tests/unit/test1_unit_tests/src/main.c
@@ -887,22 +887,33 @@ ZTEST(zcbor_unit_tests, test_simple)
 	ZCBOR_STATE_D(state_d, 1, payload1, sizeof(payload1), 16, 0);
 	uint8_t simple1 = 0;
 	uint8_t simple2 = 2;
+	uint8_t simple3 = 0;
 
 	zassert_true(zcbor_simple_encode(state_e, &simple1), NULL);
 	zassert_true(zcbor_simple_put(state_e, 14), NULL);
 	zassert_true(zcbor_simple_put(state_e, 22), NULL);
-	zassert_true(zcbor_simple_put(state_e, 24), NULL);
+	zassert_false(zcbor_simple_put(state_e, 24), NULL);
+	zassert_equal(ZCBOR_ERR_INVALID_VALUE_ENCODING, zcbor_peek_error(state_e), NULL);
+	zassert_true(zcbor_simple_put(state_e, 32), NULL);
 	zassert_true(zcbor_simple_put(state_e, 255), NULL);
+	state_e->payload_mut[0] = 0xF8;
+	state_e->payload_mut[1] = 24; // Invalid value
+	state_e->payload_end += 2;
 
 	zassert_true(zcbor_simple_decode(state_d, &simple2), NULL);
 	zassert_true(zcbor_simple_expect(state_d, 14), NULL);
 	zassert_true(zcbor_nil_expect(state_d, NULL), NULL);
 	zassert_false(zcbor_undefined_expect(state_d, NULL), NULL);
-	zassert_true(zcbor_simple_expect(state_d, 24), NULL);
+	zassert_true(zcbor_simple_expect(state_d, 32), NULL);
 	zassert_false(zcbor_simple_expect(state_d, 254), NULL);
 	zassert_true(zcbor_simple_decode(state_d, &simple1), NULL);
 	zassert_equal(0, simple2, NULL);
 	zassert_equal(255, simple1, NULL);
+
+	zassert_false(zcbor_simple_expect(state_d, 24), NULL);
+	zassert_equal(ZCBOR_ERR_INVALID_VALUE_ENCODING, zcbor_peek_error(state_d), "%s\n", zcbor_error_str(zcbor_peek_error(state_d)));
+	zassert_false(zcbor_simple_decode(state_d, &simple3), NULL);
+	zassert_equal(ZCBOR_ERR_INVALID_VALUE_ENCODING, zcbor_peek_error(state_d), "%s\n", zcbor_error_str(zcbor_peek_error(state_d)));
 }