diff --git a/include/trx/trx.tpp b/include/trx/trx.tpp
index e2bd7a9..3a9dd8c 100644
--- a/include/trx/trx.tpp
+++ b/include/trx/trx.tpp
@@ -359,7 +359,7 @@ TrxFile<DT>::_create_trx_from_pointer(json header,
 
       std::tuple<int, int> shape = std::make_tuple(static_cast<int>(trx->header["NB_VERTICES"].int_value()), 3);
       trx->streamlines->mmap_pos =
-          trx::_create_memmap(filename, shape, "r+", ext.substr(1, ext.size() - 1), mem_adress);
+          trx::_create_memmap(filename, shape, "r+", ext, mem_adress);
 
       trx::detail::remap(trx->streamlines->_data, trx->streamlines->mmap_pos.data(), shape);
     }
diff --git a/src/detail/dtype_helpers.cpp b/src/detail/dtype_helpers.cpp
index 8bf056c..2d07b09 100644
--- a/src/detail/dtype_helpers.cpp
+++ b/src/detail/dtype_helpers.cpp
@@ -27,7 +27,9 @@ int _sizeof_dtype(const std::string &dtype) {
     return sizeof(float);
   if (dtype == "float64")
     return sizeof(double);
-  return sizeof(std::uint16_t); // default to 16-bit float size
+  if (dtype == "float16")
+    return sizeof(std::uint16_t);
+  throw TrxDTypeError("Unrecognized dtype: " + dtype);
 }
 
 std::string _get_dtype(const std::string &dtype) {
diff --git a/tests/test_trx_mmap.cpp b/tests/test_trx_mmap.cpp
index 09e8224..b689d57 100644
--- a/tests/test_trx_mmap.cpp
+++ b/tests/test_trx_mmap.cpp
@@ -526,7 +526,8 @@ TEST(TrxFileMemmap, __sizeof_dtype_values) {
   EXPECT_EQ(trx::detail::_sizeof_dtype("int64"), sizeof(int64_t));
   EXPECT_EQ(trx::detail::_sizeof_dtype("float32"), sizeof(float));
   EXPECT_EQ(trx::detail::_sizeof_dtype("float64"), sizeof(double));
-  EXPECT_EQ(trx::detail::_sizeof_dtype("unknown"), sizeof(uint16_t));
+  EXPECT_EQ(trx::detail::_sizeof_dtype("float16"), sizeof(uint16_t));
+  EXPECT_THROW(trx::detail::_sizeof_dtype("unknown"), trx::TrxDTypeError);
 }
 
 // asserts dtype code mapping.