diff --git a/src/math/big/int_test.go b/src/math/big/int_test.go
index a972a7249b..fa4ae2d311 100644
--- a/src/math/big/int_test.go
+++ b/src/math/big/int_test.go
@@ -525,6 +525,7 @@ var expTests = []struct {
 	{"1234", "-1", "1", "0"},
 
 	// misc
+	{"5", "1", "3", "2"},
 	{"5", "-7", "", "1"},
 	{"-5", "-7", "", "1"},
 	{"5", "0", "", "1"},
diff --git a/src/math/big/nat.go b/src/math/big/nat.go
index 2a279d186c..7157a5487b 100644
--- a/src/math/big/nat.go
+++ b/src/math/big/nat.go
@@ -888,6 +888,13 @@ func (z nat) expNN(x, y, m nat) nat {
 	}
 	// y > 0
 
+	// x**1 mod m == x mod m
+	if len(y) == 1 && y[0] == 1 && len(m) != 0 {
+		_, z = z.div(z, x, m)
+		return z
+	}
+	// y > 1
+
 	if len(m) != 0 {
 		// We likely end up being as long as the modulus.
 		z = z.make(len(m))