diff --git a/library/string.m b/library/string.m
index 4c7e3df1b..9bd36a332 100644
--- a/library/string.m
+++ b/library/string.m
@@ -1269,7 +1269,9 @@ make_format(Flags, MaybeWidth, MaybePrec, LengthMod, Spec) = String :-
 :- func format_float(string, float) = string.
 :- pragma c_code(format_float(FormatStr::in, Val::in) = (Str::out),
 		[will_not_call_mercury, thread_safe], "{
+	save_transient_hp();
 	Str = MR_make_string(MR_PROC_LABEL, FormatStr, (long double) Val);
+	restore_transient_hp();
 }").
 
 	% Create a string from a int using the format string.
@@ -1278,7 +1280,9 @@ make_format(Flags, MaybeWidth, MaybePrec, LengthMod, Spec) = String :-
 :- func format_int(string, int) = string.
 :- pragma c_code(format_int(FormatStr::in, Val::in) = (Str::out),
 		[will_not_call_mercury, thread_safe], "{
+	save_transient_hp();
 	Str = MR_make_string(MR_PROC_LABEL, FormatStr, Val);
+	restore_transient_hp();
 }").
 
 	% Create a string from a string using the format string.
@@ -1296,7 +1300,9 @@ make_format(Flags, MaybeWidth, MaybePrec, LengthMod, Spec) = String :-
 :- func format_char(string, char) = string.
 :- pragma c_code(format_char(FormatStr::in, Val::in) = (Str::out),
 		[will_not_call_mercury, thread_safe], "{
+	save_transient_hp();
 	Str = MR_make_string(MR_PROC_LABEL, FormatStr, Val);
+	restore_transient_hp();
 }").
 
 %-----------------------------------------------------------------------------%
diff --git a/runtime/mercury_string.c b/runtime/mercury_string.c
index 529bf7295..30943816b 100644
--- a/runtime/mercury_string.c
+++ b/runtime/mercury_string.c
@@ -74,9 +74,10 @@ MR_make_string(MR_Code *proclabel, const char *fmt, ...) {
 
 	p = fixed;
 #endif
-	      
+	restore_transient_hp();      
 	MR_allocate_aligned_string_msg(result, strlen(p),
 			proclabel);
+	save_transient_hp();
 	strcpy(result, p);
 
 #ifdef HAVE_VSNPRINTF
diff --git a/runtime/mercury_string.h b/runtime/mercury_string.h
index fafba31ec..a5159b0b7 100644
--- a/runtime/mercury_string.h
+++ b/runtime/mercury_string.h
@@ -174,10 +174,10 @@ int	MR_hash_string(MR_Word);
 ** allocation as coming from proclabel.  The MR_String returned has been
 ** allocated on the mercury heap using MR_allocate_aligned_string_msg.
 **
-** BEWARE: this may modify `hp', so it must only be called from
-** places where `hp' is valid.  If calling it from inside a C function,
-** rather than inside Mercury code, you may need to call
-** save/restore_transient_hp().
+** BEWARE: this may modify the saved copy of `hp', so it must only be
+** called from places where the saved copy of `hp' is valid.
+** You will generally need to call save/restore_transient_hp()
+** before/after calling this function.
 */
 MR_String MR_make_string(MR_Code *proclabel, const char *fmt, ...);