diff --git a/sim/rx/ChangeLog b/sim/rx/ChangeLog
index b89926950a..320d164e9d 100644
--- a/sim/rx/ChangeLog
+++ b/sim/rx/ChangeLog
@@ -1,3 +1,8 @@
+2021-02-08  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* reg.c (fpsw2str): Provide a format string to printf.
+	(trace_register_changes): Likewise.
+
 2021-02-08  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* err.c (execution_error_exit_all): Delete.
diff --git a/sim/rx/reg.c b/sim/rx/reg.c
index 00796a0c99..ce9be12649 100644
--- a/sim/rx/reg.c
+++ b/sim/rx/reg.c
@@ -509,7 +509,7 @@ fpsw2str(int rpsw)
 #define TRC(f,n) \
   if (oldregs.f != regs.f) \
     { \
-      if (tag) { printf (tag); tag = 0; } \
+      if (tag) { printf ("%s", tag); tag = 0; }  \
       printf("  %s %08x:%08x", n, \
 	     (unsigned int)oldregs.f, \
 	     (unsigned int)regs.f); \
@@ -531,7 +531,7 @@ trace_register_changes (void)
   TRC (r_isp, "isp");
   if (oldregs.r_psw != regs.r_psw)
     {
-      if (tag) { printf (tag); tag = 0; }
+      if (tag) { printf ("%s", tag); tag = 0; }
       printf("  psw %s:", psw2str(oldregs.r_psw));
       printf("%s", psw2str(regs.r_psw));
       oldregs.r_psw = regs.r_psw;
@@ -539,7 +539,7 @@ trace_register_changes (void)
 
   if (oldregs.r_fpsw != regs.r_fpsw)
     {
-      if (tag) { printf (tag); tag = 0; }
+      if (tag) { printf ("%s", tag); tag = 0; }
       printf("  fpsw %s:", fpsw2str(oldregs.r_fpsw));
       printf("%s", fpsw2str(regs.r_fpsw));
       oldregs.r_fpsw = regs.r_fpsw;
@@ -547,7 +547,7 @@ trace_register_changes (void)
 
   if (oldregs.r_acc != regs.r_acc)
     {
-      if (tag) { printf (tag); tag = 0; }
+      if (tag) { printf ("%s", tag); tag = 0; }
       printf("  acc %016llx:", oldregs.r_acc);
       printf("%016llx", regs.r_acc);
       oldregs.r_acc = regs.r_acc;