diff --git a/src/Init/Init_FFTW.cpp b/src/Init/Init_FFTW.cpp
index 4735ddd78..dabb0b238 100644
--- a/src/Init/Init_FFTW.cpp
+++ b/src/Init/Init_FFTW.cpp
@@ -358,11 +358,13 @@ void Patch2Slab( real *VarS, real *SendBuf_Var, real *RecvBuf_Var, long *SendBuf
 #  endif // GAMER_DEBUG
 
 
-   const int SSize[2]   = { ( InPlacePad ? 2*(FFT_Size[0]/2+1) : FFT_Size[0] ), FFT_Size[1] };  // padded slab size in the x and y directions
-   const int PSSize     = PS1*PS1;                          // patch slice size
-   const int MemUnit    = amr->NPatchComma[0][1]/MPI_NRank; // set arbitrarily; divided by MPI_NRank so that the memory consumption per rank for arrays
-                                                            // like TempBuf_Var[MPI_NRank] reduces when launching more ranks
-   const int AveNz      = FFT_Size[2]/MPI_NRank + ( ( FFT_Size[2]%MPI_NRank == 0 ) ? 0 : 1 );   // average slab thickness
+   const int SSize[2]   = { ( InPlacePad ? 2*(FFT_Size[0]/2+1) : FFT_Size[0] ), FFT_Size[1] }; // padded slab size in the x and y directions
+   const int PSSize     = PS1*PS1;                                                             // patch slice size
+   const int MemUnit    = MAX( 1, amr->NPatchComma[0][1]/MPI_NRank );                          // set arbitrarily; divided by MPI_NRank so that the
+                                                                                               // memory consumption per rank for arrays like
+                                                                                               // TempBuf_Var[MPI_NRank] reduces when launching
+                                                                                               // more ranks
+   const int AveNz      = FFT_Size[2]/MPI_NRank + ( ( FFT_Size[2]%MPI_NRank == 0 ) ? 0 : 1 );  // average slab thickness
    const int Scale0     = amr->scale[0];
 
    int   Cr[3];                        // corner coordinates of each patch normalized to the base-level grid size