diff --git a/diff-cache.c b/diff-cache.c
index b47b45a40d2..c6043594014 100644
--- a/diff-cache.c
+++ b/diff-cache.c
@@ -8,7 +8,7 @@ static int line_termination = '\n';
 static int detect_rename = 0;
 static int reverse_diff = 0;
 static int diff_score_opt = 0;
-static char *pickaxe = 0;
+static const char *pickaxe = NULL;
 
 /* A file entry went away or appeared */
 static void show_file(const char *prefix, struct cache_entry *ce, unsigned char *sha1, unsigned int mode)
diff --git a/diff-files.c b/diff-files.c
index a3c523ff6fe..6dca9e0e2b5 100644
--- a/diff-files.c
+++ b/diff-files.c
@@ -14,7 +14,7 @@ static int line_termination = '\n';
 static int detect_rename = 0;
 static int reverse_diff = 0;
 static int diff_score_opt = 0;
-static char *pickaxe = 0;
+static const char *pickaxe = NULL;
 static int silent = 0;
 
 static int matches_pathspec(struct cache_entry *ce, char **spec, int cnt)
diff --git a/diff-helper.c b/diff-helper.c
index 568d5ae356c..3c39c96d7e8 100644
--- a/diff-helper.c
+++ b/diff-helper.c
@@ -9,7 +9,7 @@
 static int detect_rename = 0;
 static int diff_score_opt = 0;
 static int generate_patch = 1;
-static char *pickaxe = 0;
+static const char *pickaxe = NULL;
 
 static int parse_oneside_change(const char *cp, int *mode,
 				unsigned char *sha1, char *path)
diff --git a/diff-tree.c b/diff-tree.c
index 868404f7c40..73aa1496e8d 100644
--- a/diff-tree.c
+++ b/diff-tree.c
@@ -13,7 +13,7 @@ static int generate_patch = 0;
 static int detect_rename = 0;
 static int reverse_diff = 0;
 static int diff_score_opt = 0;
-static char *pickaxe = 0;
+static const char *pickaxe = NULL;
 static const char *header = NULL;
 static const char *header_prefix = "";
 
diff --git a/diff.c b/diff.c
index 13d5e3edf4f..f7d4bc4b5aa 100644
--- a/diff.c
+++ b/diff.c
@@ -513,7 +513,7 @@ int diff_scoreopt_parse(const char *opt)
 }
 
 void diff_setup(int detect_rename_, int minimum_score_,
-		char *pickaxe_,
+		const char *pickaxe_,
 		int reverse_diff_, int diff_raw_output_,
 		const char **pathspec_, int speccnt_)
 {
diff --git a/diff.h b/diff.h
index 97d39ac9617..7473c522846 100644
--- a/diff.h
+++ b/diff.h
@@ -20,7 +20,7 @@ extern void diff_unmerge(const char *path);
 extern int diff_scoreopt_parse(const char *opt);
 
 extern void diff_setup(int detect_rename, int minimum_score,
-		       char *pickaxe,
+		       const char *pickaxe,
 		       int reverse, int raw_output,
 		       const char **spec, int cnt);