diff --git a/Makefile b/Makefile
index 3aafbc38..a8bf80de 100644
--- a/Makefile
+++ b/Makefile
@@ -43,6 +43,12 @@ endif
 # Run the unit tests.
 test : all test_functional test_unit
 
+# Test for C++ const correctness on APIs.
+test_const : all
+	$(Q)$(MAKE) clean -C test/functional/src
+	$(Q)! $(MAKE) -C test/functional/src CFLAGS=-Wwrite-strings 2>&1 | grep -A 1 "note:"
+
+
 # Run the functional tests.
 test_functional : all
 	$(Q)$(MAKE) -C test/functional/src
diff --git a/dev/release/release_check.sh b/dev/release/release_check.sh
index 6690f6bd..75c8bb80 100755
--- a/dev/release/release_check.sh
+++ b/dev/release/release_check.sh
@@ -34,6 +34,33 @@ function check_test_status {
     fi
 }
 
+#############################################################
+#
+# Run test for C++ const correctness.
+#
+function check_test_const {
+
+    echo
+    echo -n     "Is the const test passing?             [y/N]: "
+    read RESPONSE
+
+    if [ "$RESPONSE" != "y" ]; then
+
+        echo -n "    Run test now?                      [y/N]: "
+        read RESPONSE
+
+        if [ "$RESPONSE" != "y" ]; then
+            echo
+            echo -e "Please run: make test_const\n";
+            exit 1
+        else
+            echo "    Running test...";
+            make test_const
+            check_test_const
+         fi
+    fi
+}
+
 
 #############################################################
 #
@@ -269,6 +296,8 @@ function check_git_status {
 
 check_test_status
 clear
+check_test_const
+clear
 check_spellcheck
 clear
 check_docs
diff --git a/include/xlsxwriter/chart.h b/include/xlsxwriter/chart.h
index 42349c1b..7b96b90b 100644
--- a/include/xlsxwriter/chart.h
+++ b/include/xlsxwriter/chart.h
@@ -1768,7 +1768,7 @@ void chart_series_set_labels_percentage(lxw_chart_series *series);
  * For more information see @ref chart_labels.
  */
 void chart_series_set_labels_num_format(lxw_chart_series *series,
-                                        char *num_format);
+                                        const char *num_format);
 
 /**
  * @brief Set the font properties for chart data labels in a series
@@ -1983,7 +1983,7 @@ void chart_series_set_trendline_intercept(lxw_chart_series *series,
  *
  * For more information see @ref chart_trendlines.
  */
-void chart_series_set_trendline_name(lxw_chart_series *series, char *name);
+void chart_series_set_trendline_name(lxw_chart_series *series, const char *name);
 
 /**
  * @brief Set the trendline line properties for a chart data series.
@@ -2279,7 +2279,7 @@ void chart_axis_set_num_font(lxw_chart_axis *axis, lxw_chart_font *font);
  * **Axis types**: This function is applicable to to all axes types.
  *                 See @ref ww_charts_axes.
  */
-void chart_axis_set_num_format(lxw_chart_axis *axis, char *num_format);
+void chart_axis_set_num_format(lxw_chart_axis *axis, const char *num_format);
 
 /**
  * @brief Set the line properties for a chart axis.
diff --git a/src/chart.c b/src/chart.c
index 9ecfacf7..7008ec42 100644
--- a/src/chart.c
+++ b/src/chart.c
@@ -5384,7 +5384,7 @@ chart_series_set_labels_percentage(lxw_chart_series *series)
  * Set an data labels number format.
  */
 void
-chart_series_set_labels_num_format(lxw_chart_series *series, char *num_format)
+chart_series_set_labels_num_format(lxw_chart_series *series, const char *num_format)
 {
     if (!num_format)
         return;
@@ -5531,7 +5531,7 @@ chart_series_set_trendline_intercept(lxw_chart_series *series,
  * Set a line type for a series trendline.
  */
 void
-chart_series_set_trendline_name(lxw_chart_series *series, char *name)
+chart_series_set_trendline_name(lxw_chart_series *series, const char *name)
 {
     if (!name)
         return;
@@ -5687,7 +5687,7 @@ chart_axis_set_num_font(lxw_chart_axis *axis, lxw_chart_font *font)
  * Set an axis number format.
  */
 void
-chart_axis_set_num_format(lxw_chart_axis *axis, char *num_format)
+chart_axis_set_num_format(lxw_chart_axis *axis, const char *num_format)
 {
     if (!num_format)
         return;