diff --git a/CRAN_Release.cmd b/CRAN_Release.cmd
index 45eac55747..e268e202c7 100644
--- a/CRAN_Release.cmd
+++ b/CRAN_Release.cmd
@@ -166,14 +166,14 @@ tar xvf R-devel.tar.gz
 cd R-devel
 # Following R-exts#4.3.3
 # (clang 3.6.0 works but gcc 4.9.2 fails in R's distance.c:256 error: ‘*.Lubsan_data0’ not specified in enclosing parallel)
-./configure CC="clang-5.0 -fsanitize=undefined,address -fno-sanitize=float-divide-by-zero -fno-omit-frame-pointer" CFLAGS="-g -O0 -Wall -pedantic" --without-recommended-packages --disable-byte-compiled-packages
+./configure CC="clang-5.0 -fsanitize=undefined,address -fno-sanitize=float-divide-by-zero -fno-omit-frame-pointer" CFLAGS="-g -Og -Wall -pedantic" --without-recommended-packages --disable-byte-compiled-packages
 make
 alias Rdevel='~/build/R-devel/bin/R --vanilla'
 Rdevel
 install.packages("bit64")  # important to run tests using integer64
 # Skip compatibility tests with other Suggests packages; unlikely UBSAN/ASAN problems there.
 q("no")
-Rdevel CMD INSTALL ~/data.table_1.10.1.tar.gz
+Rdevel CMD INSTALL data.table_1.10.5.tar.gz
 # Check UBSAN and ASAN flags appear in compiler output above. Rdevel was compiled with
 # them (above) so should be passed through to here
 Rdevel
diff --git a/src/data.table.h b/src/data.table.h
index 56e1ea5b79..441ac087c2 100644
--- a/src/data.table.h
+++ b/src/data.table.h
@@ -1,15 +1,10 @@
 #include <R.h>
 #define USE_RINTERNALS
 #include <Rinternals.h>
-#ifdef _OPENMP
-  #include <omp.h>
-#else // so it still compiles on machines with compilers void of openmp support
-  #define omp_get_num_threads() 1
-  #define omp_get_thread_num() 0
-#endif
 // #include <signal.h> // the debugging machinery + breakpoint aidee
 // raise(SIGINT);
 #include <stdint.h> // for uint64_t rather than unsigned long long
+#include "myomp.h"
 
 // data.table depends on R>=3.0.0 when R_xlen_t was introduced
 // Before R 3.0.0, RLEN used to be switched to R_len_t as R_xlen_t wasn't available.
diff --git a/src/fread.h b/src/fread.h
index d7a4f250a3..42279a9a86 100644
--- a/src/fread.h
+++ b/src/fread.h
@@ -3,15 +3,13 @@
 #include <stdint.h>  // uint32_t
 #include <stdlib.h>  // size_t
 #include <stdbool.h> // bool
+#include "myomp.h"
 #ifdef DTPY
-  #include "myomp.h"
   #include "py_fread.h"
 #else
-  #include <omp.h>
   #include "freadR.h"
 #endif
 
-
 // Ordered hierarchy of types
 typedef enum {
   NEG = -1,       // dummy to force signed type; sign bit used for out-of-sample type bump management
diff --git a/src/fwrite.c b/src/fwrite.c
index 3715cb5ad5..24c075918b 100644
--- a/src/fwrite.c
+++ b/src/fwrite.c
@@ -17,7 +17,7 @@
 #define WRITE write
 #define CLOSE close
 #endif
-#include <omp.h>
+#include "myomp.h"
 #include "fwrite.h"
 
 #define NUM_SF   15
diff --git a/src/myomp.h b/src/myomp.h
new file mode 100644
index 0000000000..798d071e3c
--- /dev/null
+++ b/src/myomp.h
@@ -0,0 +1,9 @@
+#ifdef _OPENMP
+  #include <omp.h>
+#else
+  // for machines with compilers void of openmp support
+  #define omp_get_num_threads() 1
+  #define omp_get_thread_num() 0
+  #define omp_get_max_threads() 1
+#endif
+