diff --git a/src/output.cpp b/src/output.cpp
index f6bf3dc6d..4246fd084 100644
--- a/src/output.cpp
+++ b/src/output.cpp
@@ -63,7 +63,7 @@ unsigned char index_for_color(rgb_color_t c) {
     return c.to_term256_index();
 }
 
-static bool write_color_escape(char *todo, unsigned char idx, bool is_fg) {
+static bool write_color_escape(const char *todo, unsigned char idx, bool is_fg) {
     if (term_supports_color_natively(idx)) {
         // Use tparm to emit color escape.
         writembs(tparm((char *)todo, idx));
@@ -550,7 +550,7 @@ rgb_color_t parse_color(const env_var_t &var, bool is_background) {
 }
 
 /// Write specified multibyte string.
-void writembs_check(char *mbs, const char *mbs_name, bool critical, const char *file, long line) {
+void writembs_check(const char *mbs, const char *mbs_name, bool critical, const char *file, long line) {
     if (mbs != NULL) {
         tputs(mbs, 1, &writeb);
     } else if (critical) {
diff --git a/src/output.h b/src/output.h
index 15c0fa4fb..8ce26b598 100644
--- a/src/output.h
+++ b/src/output.h
@@ -30,7 +30,7 @@ enum {
 
 void set_color(rgb_color_t c, rgb_color_t c2);
 
-void writembs_check(char *mbs, const char *mbs_name, bool critical, const char *file, long line);
+void writembs_check(const char *mbs, const char *mbs_name, bool critical, const char *file, long line);
 #define writembs(mbs) writembs_check((mbs), #mbs, true, __FILE__, __LINE__)
 #define writembs_nofail(mbs) writembs_check((mbs), #mbs, false, __FILE__, __LINE__)
 
diff --git a/src/screen.cpp b/src/screen.cpp
index 54b65bbb9..4b0df4c44 100644
--- a/src/screen.cpp
+++ b/src/screen.cpp
@@ -566,7 +566,7 @@ static void s_write_char(screen_t *s, data_buffer_t *b, wchar_t c) {
 }
 
 /// Send the specified string through tputs and append the output to the specified buffer.
-static void s_write_mbs(data_buffer_t *b, char *s) {
+static void s_write_mbs(data_buffer_t *b, const char *s) {
     scoped_buffer_t scoped_buffer(b);
     writembs(s);
 }