diff --git a/appveyor.yml b/appveyor.yml
index f2579e7..4196c5b 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -12,6 +12,12 @@ install:
 
 # Adapt as necessary starting from here
 
+environment:
+  matrix:
+  - R_VERSION: stable
+    R_ARCH: x64
+    BIOC_USE_DEVEL: FALSE
+
 build_script:
   - travis-tool.sh install_bioc_deps
   - travis-tool.sh install_github zhengxwen/gdsfmt
diff --git a/src/genPCA.cpp b/src/genPCA.cpp
index 3678eee..12ee007 100755
--- a/src/genPCA.cpp
+++ b/src/genPCA.cpp
@@ -2,7 +2,7 @@
 //
 // genPCA.cpp: Principal Component Analysis on GWAS
 //
-// Copyright (C) 2011-2018    Xiuwen Zheng
+// Copyright (C) 2011-2019    Xiuwen Zheng
 //
 // This file is part of SNPRelate.
 //
@@ -218,7 +218,7 @@ void CProdMat_AlgArith::PCA_Detect_BlockNumSNP(int nSamp)
 	size_t Cache = GetOptimzedCache();
 	BlockNumSNP = Cache / (sizeof(double)*nSamp);
 	BlockNumSNP = (BlockNumSNP / 4) * 4;
-	if (BlockNumSNP < 16) BlockNumSNP = 16;
+	if (BlockNumSNP < 64) BlockNumSNP = 64;
 }
 
 // time-consuming function