diff --git a/src/halfvec.c b/src/halfvec.c
index 7aefdc1..b6173a1 100644
--- a/src/halfvec.c
+++ b/src/halfvec.c
@@ -731,9 +731,11 @@ Datum
 vector_to_halfvec(PG_FUNCTION_ARGS)
 {
 	Vector	   *vec = PG_GETARG_VECTOR_P(0);
+	int32		typmod = PG_GETARG_INT32(1);
 	HalfVector *result;
 
 	CheckDim(vec->dim);
+	CheckExpectedDim(typmod, vec->dim);
 
 	result = InitHalfVector(vec->dim);
 
diff --git a/src/vector.c b/src/vector.c
index f719a8f..1114932 100644
--- a/src/vector.c
+++ b/src/vector.c
@@ -541,9 +541,11 @@ Datum
 halfvec_to_vector(PG_FUNCTION_ARGS)
 {
 	HalfVector *vec = PG_GETARG_HALFVEC_P(0);
+	int32		typmod = PG_GETARG_INT32(1);
 	Vector	   *result;
 
 	CheckDim(vec->dim);
+	CheckExpectedDim(typmod, vec->dim);
 
 	result = InitVector(vec->dim);
 
diff --git a/test/expected/cast.out b/test/expected/cast.out
index f2e1ce7..5874714 100644
--- a/test/expected/cast.out
+++ b/test/expected/cast.out
@@ -58,6 +58,10 @@ SELECT '[1,2,3]'::halfvec::vector;
  [1,2,3]
 (1 row)
 
+SELECT '[1,2,3]'::vector::halfvec(2);
+ERROR:  expected 2 dimensions, not 3
+SELECT '[1,2,3]'::halfvec::vector(2);
+ERROR:  expected 2 dimensions, not 3
 SELECT '[1e-8]'::vector::halfvec;
  halfvec 
 ---------
diff --git a/test/sql/cast.sql b/test/sql/cast.sql
index b86f4be..b85dadd 100644
--- a/test/sql/cast.sql
+++ b/test/sql/cast.sql
@@ -12,6 +12,8 @@ SELECT '{{1}}'::real[]::vector;
 SELECT '[1,2,3]'::vector::real[];
 SELECT '[1,2,3]'::vector::halfvec;
 SELECT '[1,2,3]'::halfvec::vector;
+SELECT '[1,2,3]'::vector::halfvec(2);
+SELECT '[1,2,3]'::halfvec::vector(2);
 SELECT '[1e-8]'::vector::halfvec;
 SELECT array_agg(n)::vector FROM generate_series(1, 16001) n;
 SELECT array_to_vector(array_agg(n), 16001, false) FROM generate_series(1, 16001) n;