diff --git a/pkg/go.sum b/pkg/go.sum
index 39b2adca2..9383dbefb 100644
--- a/pkg/go.sum
+++ b/pkg/go.sum
@@ -432,6 +432,7 @@ github.com/src-d/gcfg v1.4.0 h1:xXbNR5AlLSA315x2UO+fTSSAXCDf+Ar38/6oyGbDKQ4=
 github.com/src-d/gcfg v1.4.0/go.mod h1:p/UMsR43ujA89BJY9duynAwIpvqEujIH/jFlfL7jWoI=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/objx v0.1.1/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
+github.com/stretchr/objx v0.2.0 h1:Hbg2NidpLE8veEBkEZTL3CvlkUIVzuU9jDplZO54c48=
 github.com/stretchr/objx v0.2.0/go.mod h1:qt09Ya8vawLte6SNmTgCsAVtYtaKzEcn8ATUoHMkEqE=
 github.com/stretchr/testify v1.2.1/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
diff --git a/pkg/util/buildutil/semver.go b/pkg/util/buildutil/semver.go
index b43d2b844..152cb282c 100644
--- a/pkg/util/buildutil/semver.go
+++ b/pkg/util/buildutil/semver.go
@@ -56,28 +56,28 @@ func PyPiVersionFromNpmVersion(s string) (string, error) {
 		return b.String(), nil
 	case rcVersionRegex.MatchString(s):
 		capMap := captureToMap(rcVersionRegex, s)
-		mustFprintf(&b, "%s.rc%s", capMap["version"], capMap["rcN"])
+		mustFprintf(&b, "%s-rc%s", capMap["version"], capMap["rcN"])
 		if capMap["dirty"] != "" {
 			mustFprintf(&b, "+dirty")
 		}
 		return b.String(), nil
 	case betaVersionRegex.MatchString(s):
 		capMap := captureToMap(betaVersionRegex, s)
-		mustFprintf(&b, "%s.b%s", capMap["version"], capMap["betaN"])
+		mustFprintf(&b, "%s-b%s", capMap["version"], capMap["betaN"])
 		if capMap["dirty"] != "" {
 			mustFprintf(&b, "+dirty")
 		}
 		return b.String(), nil
 	case alphaVersionRegex.MatchString(s):
 		capMap := captureToMap(alphaVersionRegex, s)
-		mustFprintf(&b, "%s.a%s", capMap["version"], capMap["time"])
+		mustFprintf(&b, "%s-a%s", capMap["version"], capMap["time"])
 		if capMap["dirty"] != "" {
 			mustFprintf(&b, "+dirty")
 		}
 		return b.String(), nil
 	case devVersionRegex.MatchString(s):
 		capMap := captureToMap(devVersionRegex, s)
-		mustFprintf(&b, "%s.dev%s", capMap["version"], capMap["time"])
+		mustFprintf(&b, "%s-dev%s", capMap["version"], capMap["time"])
 		if capMap["dirty"] != "" {
 			mustFprintf(&b, "+dirty")
 		}
diff --git a/pkg/util/buildutil/semver_test.go b/pkg/util/buildutil/semver_test.go
index 85410b454..0d85add6f 100644
--- a/pkg/util/buildutil/semver_test.go
+++ b/pkg/util/buildutil/semver_test.go
@@ -17,6 +17,7 @@ package buildutil
 import (
 	"testing"
 
+	"github.com/blang/semver"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -24,16 +25,18 @@ func TestVersions(t *testing.T) {
 	cases := map[string]string{
 		"v0.12.0":                                  "0.12.0",
 		"v0.12.0+dirty":                            "0.12.0+dirty",
-		"v0.12.0-rc.1":                             "0.12.0.rc1",
-		"v0.12.0-rc.1+dirty":                       "0.12.0.rc1+dirty",
-		"v0.12.1-dev.1524606809+gf2f1178b":         "0.12.1.dev1524606809",
-		"v0.12.1-dev.1524606809+gf2f1178b.dirty":   "0.12.1.dev1524606809+dirty",
-		"v1.14.0-alpha.1584648098+gce192898.dirty": "1.14.0.a1584648098+dirty",
+		"v0.12.0-rc.1":                             "0.12.0-rc1",
+		"v0.12.0-rc.1+dirty":                       "0.12.0-rc1+dirty",
+		"v0.12.1-dev.1524606809+gf2f1178b":         "0.12.1-dev1524606809",
+		"v0.12.1-dev.1524606809+gf2f1178b.dirty":   "0.12.1-dev1524606809+dirty",
+		"v1.14.0-alpha.1584648098+gce192898.dirty": "1.14.0-a1584648098+dirty",
 	}
 
 	for ver, expected := range cases {
 		p, err := PyPiVersionFromNpmVersion(ver)
 		assert.NoError(t, err)
 		assert.Equal(t, expected, p, "failed parsing '%s'", ver)
+		_, err = semver.ParseTolerant(p)
+		assert.NoError(t, err)
 	}
 }