diff --git a/src/gfx.cpp b/src/gfx.cpp
index e698d92546..9cb199e2df 100644
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -653,7 +653,7 @@ int DrawString(int left, int right, int top, StringID str, TextColour colour, St
  *  0 - 15 the number of lines ADDED to the string
  * 16 - 31 the fontsize in which the length calculation was done at
  */
-uint32 FormatStringLinebreaks(char *str, const char *last, int maxw, FontSize size)
+static uint32 FormatStringLinebreaks(char *str, const char *last, int maxw, FontSize size = FS_NORMAL)
 {
 	int num = 0;
 
diff --git a/src/gfx_func.h b/src/gfx_func.h
index 88d6e9336f..6ec36ee1b1 100644
--- a/src/gfx_func.h
+++ b/src/gfx_func.h
@@ -121,7 +121,6 @@ void DrawBox(int x, int y, int dx1, int dy1, int dx2, int dy2, int dx3, int dy3)
 
 Dimension GetStringBoundingBox(const char *str, FontSize start_fontsize = FS_NORMAL);
 Dimension GetStringBoundingBox(StringID strid);
-uint32 FormatStringLinebreaks(char *str, const char *last, int maxw, FontSize start_fontsize = FS_NORMAL);
 int GetStringHeight(StringID str, int maxw);
 int GetStringLineCount(StringID str, int maxw);
 Dimension GetStringMultiLineBoundingBox(StringID str, const Dimension &suggestion);