diff --git a/dlls/oledb32/convert.c b/dlls/oledb32/convert.c
index 8525c6c571a..f2c4c29bad5 100644
--- a/dlls/oledb32/convert.c
+++ b/dlls/oledb32/convert.c
@@ -1108,6 +1108,12 @@ static HRESULT WINAPI convert_GetConversionSize(IDataConvert* iface,
     {
         switch (src_type)
         {
+        case DBTYPE_STR:
+            if(src_len)
+                *dst_len = (*src_len + 1) * sizeof(WCHAR);
+            else
+                *dst_len = (strlen(src) + 1) * sizeof(WCHAR);
+            break;
         case DBTYPE_WSTR:
             if(src_len)
                 *dst_len = (*src_len) + sizeof(WCHAR);
diff --git a/dlls/oledb32/tests/convert.c b/dlls/oledb32/tests/convert.c
index d1f6c4ed6d0..51dc0f6e132 100644
--- a/dlls/oledb32/tests/convert.c
+++ b/dlls/oledb32/tests/convert.c
@@ -2484,6 +2484,7 @@ static void test_getconversionsize(void)
     HRESULT hr;
     BSTR str;
     static WCHAR strW[] = {'t','e','s','t',0};
+    static char strTest[] = "test";
 
     hr = CoCreateInstance(&CLSID_OLEDB_CONVERSIONLIBRARY, NULL, CLSCTX_INPROC_SERVER, &IID_IDataConvert, (void**)&convert);
     if(FAILED(hr))
@@ -2530,6 +2531,23 @@ static void test_getconversionsize(void)
     ok(hr == S_OK, "got 0x%08x\n", hr);
     ok(dst_len == 22, "%ld\n", dst_len);
 
+    dst_len = 0;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_STR, DBTYPE_WSTR, NULL, &dst_len, strTest);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(dst_len == 10 || broken(dst_len == 12), "%ld\n", dst_len);
+
+    dst_len = 0;
+    src_len = 2;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_STR, DBTYPE_WSTR, &src_len, &dst_len, strTest);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(dst_len == 6, "%ld\n", dst_len);
+
+    dst_len = 0;
+    src_len = 20;
+    hr = IDataConvert_GetConversionSize(convert, DBTYPE_STR, DBTYPE_WSTR, &src_len, &dst_len, strTest);
+    ok(hr == S_OK, "got 0x%08x\n", hr);
+    ok(dst_len == 42, "%ld\n", dst_len);
+
     IDataConvert_Release(convert);
 }