diff --git a/dlls/oledb32/convert.c b/dlls/oledb32/convert.c
index 891b31c1bd5..0a1518d9a92 100644
--- a/dlls/oledb32/convert.c
+++ b/dlls/oledb32/convert.c
@@ -1344,6 +1344,9 @@ static HRESULT WINAPI convert_GetConversionSize(IDataConvert* iface,
     if ((*dst_len = get_length(dst_type)))
         return S_OK;
 
+    if(src_type == DBTYPE_VARIANT && V_VT((VARIANT*)src) == VT_NULL)
+        return S_OK;
+
     switch (dst_type)
     {
     case DBTYPE_STR:
diff --git a/dlls/oledb32/tests/convert.c b/dlls/oledb32/tests/convert.c
index a7854577a3a..46719b19197 100644
--- a/dlls/oledb32/tests/convert.c
+++ b/dlls/oledb32/tests/convert.c
@@ -2631,16 +2631,25 @@ static void test_getconversionsize(void)
     src_len = 20;
     V_VT(&var) = VT_BSTR;
     V_BSTR(&var) = SysAllocString(strW);
-    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_BYTES, &src_len, &dst_len, &var);
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_STR, &src_len, &dst_len, &var);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(dst_len == 5, "%ld\n", dst_len);
+    VariantClear(&var);
+
+    src_len = 20;
+    V_VT(&var) = VT_I4;
+    V_I4(&var) = 4;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_STR, &src_len, &dst_len, &var);
     ok(hr == S_OK, "got 0x%08x\n", hr);
-    ok(dst_len == 2, "%ld\n", dst_len);
     VariantClear(&var);
 
     dst_len = 0;
     src_len = 20;
-    V_VT(&var) = VT_NULL;
+    V_VT(&var) = VT_BSTR;
+    V_BSTR(&var) = SysAllocString(strW);
     hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_BYTES, &src_len, &dst_len, &var);
     ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(dst_len == 2, "%ld\n", dst_len);
     VariantClear(&var);
 
     dst_len = 0;
@@ -2655,6 +2664,30 @@ static void test_getconversionsize(void)
     ok(hr == S_OK, "got 0x%08x\n", hr);
     ok(dst_len == 1802, "%ld\n", dst_len);
     VariantClear(&var);
+
+    /* On Windows, NULL variants being convert to a non-fixed sized type will return a dst_len of
+     * 110 but we aren't testing for this value.
+     */
+    dst_len = 32;
+    V_VT(&var) = VT_NULL;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_I4, NULL, &dst_len, &var);
+    ok(dst_len == 4, "%ld\n", dst_len);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+
+    V_VT(&var) = VT_NULL;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_STR, NULL, &dst_len, &var);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+
+    V_VT(&var) = VT_NULL;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_WSTR, NULL, &dst_len, &var);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+
+    src_len = 20;
+    V_VT(&var) = VT_NULL;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_VARIANT, DBTYPE_BYTES, &src_len, &dst_len, &var);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    VariantClear(&var);
+
 }
 
 static void test_converttobytes(void)