diff --git a/dll/win32/msi/install.c b/dll/win32/msi/install.c
index ba329eb8f80..88b3ff518e2 100644
--- a/dll/win32/msi/install.c
+++ b/dll/win32/msi/install.c
@@ -334,50 +334,19 @@ static UINT MSI_GetSourcePath( MSIHANDLE hInstall, LPCWSTR szFolder,
     package = msihandle2msiinfo( hInstall, MSIHANDLETYPE_PACKAGE );
     if (!package)
     {
-        HRESULT hr;
         LPWSTR value = NULL;
         MSIHANDLE remote;
-        BSTR folder;
-        DWORD len;
 
         if (!(remote = msi_get_remote(hInstall)))
             return ERROR_INVALID_HANDLE;
 
-        folder = SysAllocString( szFolder );
-        if (!folder)
-            return ERROR_OUTOFMEMORY;
+        r = remote_GetSourcePath(remote, szFolder, &value);
+        if (r != ERROR_SUCCESS)
+            return r;
 
-        len = 0;
-        hr = remote_GetSourcePath(remote, folder, NULL, &len);
-        if (FAILED(hr))
-            goto done;
-
-        len++;
-        value = msi_alloc(len * sizeof(WCHAR));
-        if (!value)
-        {
-            r = ERROR_OUTOFMEMORY;
-            goto done;
-        }
-
-        hr = remote_GetSourcePath(remote, folder, value, &len);
-        if (FAILED(hr))
-            goto done;
-
-        r = msi_strcpy_to_awstring( value, len, szPathBuf, pcchPathBuf );
-
-done:
-        SysFreeString( folder );
-        msi_free( value );
-
-        if (FAILED(hr))
-        {
-            if (HRESULT_FACILITY(hr) == FACILITY_WIN32)
-                return HRESULT_CODE(hr);
-
-            return ERROR_FUNCTION_FAILED;
-        }
+        r = msi_strcpy_to_awstring(value, -1, szPathBuf, pcchPathBuf);
 
+        midl_user_free(value);
         return r;
     }
 
diff --git a/dll/win32/msi/package.c b/dll/win32/msi/package.c
index 89ac2ce3a6f..1e554af5eee 100644
--- a/dll/win32/msi/package.c
+++ b/dll/win32/msi/package.c
@@ -2503,10 +2503,21 @@ UINT __cdecl remote_SetTargetPath(MSIHANDLE hinst, LPCWSTR folder, LPCWSTR value
     return MsiSetTargetPathW(hinst, folder, value);
 }
 
-HRESULT __cdecl remote_GetSourcePath(MSIHANDLE hinst, BSTR folder, BSTR value, DWORD *size)
+UINT __cdecl remote_GetSourcePath(MSIHANDLE hinst, LPCWSTR folder, LPWSTR *value)
 {
-    UINT r = MsiGetSourcePathW(hinst, folder, value, size);
-    return HRESULT_FROM_WIN32(r);
+    WCHAR empty[1];
+    DWORD size = 1;
+    UINT r;
+
+    r = MsiGetSourcePathW(hinst, folder, empty, &size);
+    if (r == ERROR_MORE_DATA)
+    {
+        *value = midl_user_allocate(++size * sizeof(WCHAR));
+        if (!*value)
+            return ERROR_OUTOFMEMORY;
+        r = MsiGetSourcePathW(hinst, folder, *value, &size);
+    }
+    return r;
 }
 
 HRESULT __cdecl remote_GetMode(MSIHANDLE hinst, MSIRUNMODE mode, BOOL *ret)
diff --git a/dll/win32/msi/winemsi.idl b/dll/win32/msi/winemsi.idl
index a0e1ddbb840..06db52e5fc5 100644
--- a/dll/win32/msi/winemsi.idl
+++ b/dll/win32/msi/winemsi.idl
@@ -78,7 +78,7 @@ interface IWineMsiRemote
     UINT remote_Sequence( [in] MSIHANDLE hinst, [in, string] LPCWSTR table, [in] int sequence );
     UINT remote_GetTargetPath( [in] MSIHANDLE hinst, [in, string] LPCWSTR folder, [out, string] LPWSTR *value );
     UINT remote_SetTargetPath( [in] MSIHANDLE hinst, [in, string] LPCWSTR folder, [in, string] LPCWSTR value );
-    HRESULT remote_GetSourcePath( [in] MSIHANDLE hinst, [in] BSTR folder, [out, size_is(*size)] BSTR value, [in, out] DWORD *size );
+    UINT remote_GetSourcePath( [in] MSIHANDLE hinst, [in, string] LPCWSTR folder, [out, string] LPWSTR *value );
     HRESULT remote_GetMode( [in] MSIHANDLE hinst, [in] MSIRUNMODE mode, [out] BOOL *ret );
     HRESULT remote_SetMode( [in] MSIHANDLE hinst, [in] MSIRUNMODE mode, [in] BOOL state );
     HRESULT remote_GetFeatureState( [in] MSIHANDLE hinst, [in] BSTR feature, [out] INSTALLSTATE *installed, [out] INSTALLSTATE *action );
diff --git a/modules/rostests/winetests/msi/custom.c b/modules/rostests/winetests/msi/custom.c
index 111032c37a4..d596f1ec328 100644
--- a/modules/rostests/winetests/msi/custom.c
+++ b/modules/rostests/winetests/msi/custom.c
@@ -458,9 +458,9 @@ static void test_targetpath(MSIHANDLE hinst)
     static const WCHAR targetdirW[] = {'T','A','R','G','E','T','D','I','R',0};
     static const WCHAR xyzW[] = {'C',':','\\',0};
     static const WCHAR xyW[] = {'C',':',0};
-    char buffer[20];
-    WCHAR bufferW[20];
-    DWORD sz;
+    WCHAR bufferW[100];
+    char buffer[100];
+    DWORD sz, srcsz;
     UINT r;
 
     /* test invalid values */
@@ -557,6 +557,89 @@ static void test_targetpath(MSIHANDLE hinst)
     ok(hinst, !strcmp(buffer, "C:\\subdir\\"), "got \"%s\"\n", buffer);
 
     r = MsiSetTargetPathA(hinst, "TARGETDIR", "C:\\");
+
+    /* test GetSourcePath() */
+
+    r = MsiGetSourcePathA(hinst, NULL, NULL, NULL);
+    ok(hinst, r == ERROR_INVALID_PARAMETER, "got %u\n", r);
+
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", NULL, NULL );
+    ok(hinst, !r, "got %u\n", r);
+
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", buffer, NULL );
+    ok(hinst, r == ERROR_INVALID_PARAMETER, "got %u\n", r);
+
+    /* Returned size is in bytes, not chars, but only for custom actions.
+     * Seems to be a casualty of RPC... */
+
+    srcsz = 0;
+    MsiGetSourcePathW(hinst, targetdirW, NULL, &srcsz);
+
+    sz = 0;
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", NULL, &sz);
+    ok(hinst, !r, "got %u\n", r);
+    todo_wine_ok(hinst, sz == srcsz * 2, "got size %u\n", sz);
+
+    sz = 0;
+    strcpy(buffer,"q");
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", buffer, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, !strcmp(buffer, "q"), "got \"%s\"\n", buffer);
+    todo_wine_ok(hinst, sz == srcsz * 2, "got size %u\n", sz);
+
+    sz = 1;
+    strcpy(buffer,"x");
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", buffer, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, !buffer[0], "got \"%s\"\n", buffer);
+    todo_wine_ok(hinst, sz == srcsz * 2, "got size %u\n", sz);
+
+    sz = srcsz;
+    strcpy(buffer,"x");
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", buffer, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, strlen(buffer) == srcsz - 1, "wrong buffer length %d\n", strlen(buffer));
+    todo_wine_ok(hinst, sz == srcsz * 2, "got size %u\n", sz);
+
+    sz = srcsz + 1;
+    strcpy(buffer,"x");
+    r = MsiGetSourcePathA(hinst, "TARGETDIR", buffer, &sz);
+    ok(hinst, !r, "got %u\n", r);
+    ok(hinst, strlen(buffer) == srcsz, "wrong buffer length %d\n", strlen(buffer));
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
+
+    sz = 0;
+    r = MsiGetSourcePathW(hinst, targetdirW, NULL, &sz);
+    ok(hinst, !r, "got %u\n", r);
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
+
+    sz = 0;
+    bufferW[0] = 'q';
+    r = MsiGetSourcePathW(hinst, targetdirW, bufferW, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, bufferW[0] == 'q', "got %s\n", dbgstr_w(bufferW));
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
+
+    sz = 1;
+    bufferW[0] = 'q';
+    r = MsiGetSourcePathW(hinst, targetdirW, bufferW, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, !bufferW[0], "got %s\n", dbgstr_w(bufferW));
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
+
+    sz = srcsz;
+    bufferW[0] = 'q';
+    r = MsiGetSourcePathW(hinst, targetdirW, bufferW, &sz);
+    ok(hinst, r == ERROR_MORE_DATA, "got %u\n", r);
+    ok(hinst, lstrlenW(bufferW) == srcsz - 1, "wrong buffer length %d\n", lstrlenW(bufferW));
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
+
+    sz = srcsz + 1;
+    bufferW[0] = 'q';
+    r = MsiGetSourcePathW(hinst, targetdirW, bufferW, &sz);
+    ok(hinst, !r, "got %u\n", r);
+    ok(hinst, lstrlenW(bufferW) == srcsz, "wrong buffer length %d\n", lstrlenW(bufferW));
+    ok(hinst, sz == srcsz, "got size %u\n", sz);
 }
 
 /* Main test. Anything that doesn't depend on a specific install configuration