diff --git a/dlls/d3dx9_36/mesh.c b/dlls/d3dx9_36/mesh.c
index 5e0bb98efc7..22f86f10d08 100644
--- a/dlls/d3dx9_36/mesh.c
+++ b/dlls/d3dx9_36/mesh.c
@@ -2409,20 +2409,21 @@ BOOL WINAPI D3DXIntersectTri(const D3DXVECTOR3 *p0, const D3DXVECTOR3 *p1, const
     return FALSE;
 }
 
-BOOL WINAPI D3DXSphereBoundProbe(const D3DXVECTOR3 *pcenter, float radius,
-        const D3DXVECTOR3 *prayposition, const D3DXVECTOR3 *praydirection)
+BOOL WINAPI D3DXSphereBoundProbe(const D3DXVECTOR3 *center, float radius,
+        const D3DXVECTOR3 *ray_position, const D3DXVECTOR3 *ray_direction)
 {
-    D3DXVECTOR3 difference;
-    FLOAT a, b, c, d;
+    D3DXVECTOR3 difference = {0};
+    float a, b, c, d;
 
-    a = D3DXVec3LengthSq(praydirection);
-    if (!D3DXVec3Subtract(&difference, prayposition, pcenter)) return FALSE;
-    b = D3DXVec3Dot(&difference, praydirection);
+    D3DXVec3Subtract(&difference, ray_position, center);
     c = D3DXVec3LengthSq(&difference) - radius * radius;
+    if (c < 0.0f)
+        return TRUE;
+    a = D3DXVec3LengthSq(ray_direction);
+    b = D3DXVec3Dot(&difference, ray_direction);
     d = b * b - a * c;
 
-    if ( ( d <= 0.0f ) || ( sqrt(d) <= b ) ) return FALSE;
-    return TRUE;
+    return d >= 0.0f && (b <= 0.0f || d > b * b);
 }
 
 /*************************************************************************
diff --git a/dlls/d3dx9_36/tests/mesh.c b/dlls/d3dx9_36/tests/mesh.c
index c357200656b..42deb32b934 100644
--- a/dlls/d3dx9_36/tests/mesh.c
+++ b/dlls/d3dx9_36/tests/mesh.c
@@ -435,18 +435,42 @@ static void D3DXBoundProbeTest(void)
     radius = sqrt(77.0f);
     center.x = 1.0f; center.y = 2.0f; center.z = 3.0f;
     raydirection.x = 2.0f; raydirection.y = -4.0f; raydirection.z = 2.0f;
-
     rayposition.x = 5.0f; rayposition.y = 5.0f; rayposition.z = 9.0f;
     result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
-    ok(result == TRUE, "expected TRUE, received FALSE\n");
+    ok(result == TRUE, "Got unexpected result %#x.\n", result);
 
     rayposition.x = 45.0f; rayposition.y = -75.0f; rayposition.z = 49.0f;
     result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
-    ok(result == FALSE, "expected FALSE, received TRUE\n");
+    ok(result == FALSE, "Got unexpected result %#x.\n", result);
+
+    raydirection.x = -2.0f; raydirection.y = 4.0f; raydirection.z = -2.0f;
+    result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
+    ok(result == TRUE, "Got unexpected result %#x.\n", result);
 
     rayposition.x = 5.0f; rayposition.y = 11.0f; rayposition.z = 9.0f;
     result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
-    ok(result == FALSE, "expected FALSE, received TRUE\n");
+    ok(result == FALSE, "Got unexpected result %#x.\n", result);
+
+    raydirection.x = 2.0f; raydirection.y = -4.0f; raydirection.z = 2.0f;
+    result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
+    ok(result == FALSE, "Got unexpected result %#x.\n", result);
+
+    radius = 1.0f;
+    rayposition.x = 2.0f; rayposition.y = 2.0f; rayposition.z = 3.0f;
+    result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
+    ok(result == FALSE, "Got unexpected result %#x.\n", result);
+
+    raydirection.x = 0.0f; raydirection.y = 0.0f; raydirection.z = 1.0f;
+    result = D3DXSphereBoundProbe(&center, radius, &rayposition, &raydirection);
+    ok(result == TRUE, "Got unexpected result %#x.\n", result);
+
+    if (0)
+    {
+        /* All these crash on native. */
+        D3DXSphereBoundProbe(&center, radius, &rayposition, NULL);
+        D3DXSphereBoundProbe(&center, radius, NULL, &raydirection);
+        D3DXSphereBoundProbe(NULL, radius, &rayposition, &raydirection);
+    }
 }
 
 static void D3DXComputeBoundingBoxTest(void)