diff --git a/Documentation/git-format-patch.txt b/Documentation/git-format-patch.txt
index aba4c5f..2730442 100644
--- a/Documentation/git-format-patch.txt
+++ b/Documentation/git-format-patch.txt
@@ -250,6 +250,11 @@
 	feature/v2`), or a revision range if the two versions of the series are
 	disjoint (for example `git format-patch --cover-letter
 	--range-diff=feature/v1~3..feature/v1 -3 feature/v2`).
++
+Note that diff options passed to the command affect how the primary
+product of `format-patch` is generated, and they are not passed to
+the underlying `range-diff` machinery used to generate the cover-letter
+material (this may change in the future).
 
 --creation-factor=<percent>::
 	Used with `--range-diff`, tweak the heuristic which matches up commits
diff --git a/builtin/log.c b/builtin/log.c
index 0fe6f9b..5ac18e2 100644
--- a/builtin/log.c
+++ b/builtin/log.c
@@ -1096,7 +1096,7 @@
 	if (rev->rdiff1) {
 		fprintf_ln(rev->diffopt.file, "%s", rev->rdiff_title);
 		show_range_diff(rev->rdiff1, rev->rdiff2,
-				rev->creation_factor, 1, &rev->diffopt);
+				rev->creation_factor, 1, NULL);
 	}
 }
 
diff --git a/log-tree.c b/log-tree.c
index 7a83e99..b243779 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -762,7 +762,7 @@
 		next_commentary_block(opt, NULL);
 		fprintf_ln(opt->diffopt.file, "%s", opt->rdiff_title);
 		show_range_diff(opt->rdiff1, opt->rdiff2,
-				opt->creation_factor, 1, &opt->diffopt);
+				opt->creation_factor, 1, NULL);
 
 		memcpy(&diff_queued_diff, &dq, sizeof(diff_queued_diff));
 	}
diff --git a/range-diff.c b/range-diff.c
index 767af8c..48b0e1b 100644
--- a/range-diff.c
+++ b/range-diff.c
@@ -460,7 +460,11 @@
 		struct diff_options opts;
 		struct strbuf indent = STRBUF_INIT;
 
-		memcpy(&opts, diffopt, sizeof(opts));
+		if (diffopt)
+			memcpy(&opts, diffopt, sizeof(opts));
+		else
+			diff_setup(&opts);
+
 		if (!opts.output_format)
 			opts.output_format = DIFF_FORMAT_PATCH;
 		opts.flags.suppress_diff_headers = 1;
diff --git a/range-diff.h b/range-diff.h
index 190593f..08a50b6 100644
--- a/range-diff.h
+++ b/range-diff.h
@@ -5,6 +5,11 @@
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
 
+/*
+ * Compare series of commmits in RANGE1 and RANGE2, and emit to the
+ * standard output.  NULL can be passed to DIFFOPT to use the built-in
+ * default.
+ */
 int show_range_diff(const char *range1, const char *range2,
 		    int creation_factor, int dual_color,
 		    struct diff_options *diffopt);