diff --git a/boards/sim/sim/sim/scripts/Make.defs b/boards/sim/sim/sim/scripts/Make.defs
index e865f0c7a5..22c4835d68 100644
--- a/boards/sim/sim/sim/scripts/Make.defs
+++ b/boards/sim/sim/sim/scripts/Make.defs
@@ -93,11 +93,16 @@ NM = $(CROSSDEV)nm
 OBJCOPY = $(CROSSDEV)objcopy
 OBJDUMP = $(CROSSDEV)objdump
 
+# Note: -fshort-wchar for the case where NuttX and the host OS have
+# differnt wchar_t. On Nuttx, it's uint16_t. On macOS, it's 32-bit.
 CFLAGS := $(ARCHWARNINGS) $(ARCHOPTIMIZATION) \
-   $(ARCHCPUFLAGS) $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe
+   $(ARCHCPUFLAGS) $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe \
+   -fshort-wchar
 CXXFLAGS := $(ARCHWARNINGSXX) $(ARCHOPTIMIZATION) \
-   $(ARCHCPUFLAGSXX) $(ARCHXXINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe
-CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS)
+   $(ARCHCPUFLAGSXX) $(ARCHXXINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) -pipe \
+   -fshort-wchar
+CPPFLAGS := $(ARCHINCLUDES) $(ARCHDEFINES) $(EXTRAFLAGS) \
+   -fshort-wchar
 AFLAGS := $(CFLAGS) -D__ASSEMBLY__
 
 ifeq ($(CONFIG_LIBCXX),y)