diff --git a/.travis.yml b/.travis.yml
index 24fd6e268e1..7125c3b2c61 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -36,7 +36,7 @@ env:
     - CC=mpicc
     - FC=mpif90
     - CPPFLAGS='-I/usr/include'
-    - CFLAGS='-std=c99'
+    - CFLAGS='-std=c99 -Werror'
     - LDFLAGS='-L/usr/lib'
   
 script:
diff --git a/src/flib/pio_support.F90 b/src/flib/pio_support.F90
index 20831d85f1b..ae212d1bbdc 100644
--- a/src/flib/pio_support.F90
+++ b/src/flib/pio_support.F90
@@ -166,6 +166,7 @@ integer(c_int) function PIOc_writemap_from_f90(file, ndims, gdims, maplen, map,
          integer(C_INT), value, intent(in) :: f90_comm
        end function PIOc_writemap_from_f90
     end interface
+    if (present(punit)) continue ! to suppress warning
     ndims = size(gdims)
     err = PIOc_writemap_from_f90(trim(file)//C_NULL_CHAR, ndims, gdims, int(size(dof),C_SIZE_T), dof, comm)
 
@@ -207,7 +208,7 @@ integer(C_INT) function PIOc_readmap_from_f90(file, ndims, gdims, maplen, map, f
          integer(C_INT), value, intent(in) :: f90_comm
        end function PIOc_readmap_from_f90
     end interface
-
+    if (present(punit)) continue ! to suppress warning
     ierr = PIOc_readmap_from_f90(trim(file)//C_NULL_CHAR, ndims, tgdims, maplen, tmap, comm);
 
     call c_f_pointer(tgdims, gdims, (/ndims/))