diff --git a/gogio/build_info.go b/gogio/build_info.go
index e3f4175..3dd6cdb 100644
--- a/gogio/build_info.go
+++ b/gogio/build_info.go
@@ -91,11 +91,8 @@ func (s Semver) String() string {
 func parseSemver(v string) (Semver, error) {
 	var sv Semver
 	_, err := fmt.Sscanf(v, "%d.%d.%d.%d", &sv.Major, &sv.Minor, &sv.Patch, &sv.VersionCode)
-	if err != nil {
-		return Semver{}, fmt.Errorf("invalid semver: %q", v)
-	}
-	if sv.String() != v {
-		return Semver{}, fmt.Errorf("invalid semver: %q", v)
+	if err != nil || sv.String() != v {
+		return Semver{}, fmt.Errorf("invalid semver: %q (must match major.minor.patch.versioncode)", v)
 	}
 	return sv, nil
 }
diff --git a/gogio/build_info_test.go b/gogio/build_info_test.go
index 397e2a3..03ba3f3 100644
--- a/gogio/build_info_test.go
+++ b/gogio/build_info_test.go
@@ -2,14 +2,12 @@ package main
 
 import "testing"
 
-type expval struct {
-	in, out string
-}
-
 func TestAppID(t *testing.T) {
 	t.Parallel()
 
-	tests := []expval{
+	tests := []struct {
+		in, out string
+	}{
 		{"example", "localhost.example"},
 		{"example.com", "com.example"},
 		{"www.example.com", "com.example.www"},
@@ -30,3 +28,32 @@ func TestAppID(t *testing.T) {
 		}
 	}
 }
+
+func TestVersion(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		version string
+		valid   bool
+	}{
+		{"v1", false},
+		{"v10.21.333.12", false},
+		{"1.2.3", false},
+		{"1.2.3.4", true},
+	}
+
+	for i, test := range tests {
+		ver, err := parseSemver(test.version)
+		if err != nil {
+			if test.valid {
+				t.Errorf("(%d): %q failed to parse: %v", i, test.version, err)
+			}
+			continue
+		} else if !test.valid {
+			t.Errorf("(%d): %q was unexpectedly accepted", i, test.version)
+		}
+		if got := ver.String(); got != test.version {
+			t.Errorf("(%d): %q parsed to %q", i, test.version, got)
+		}
+	}
+}
diff --git a/gogio/help.go b/gogio/help.go
index 0399e39..00ccc29 100644
--- a/gogio/help.go
+++ b/gogio/help.go
@@ -47,8 +47,9 @@ The -appid flag specifies the package name for Android or the bundle id for
 iOS and tvOS. A bundle id must be provisioned through Xcode before the gogio
 tool can use it.
 
-The -version flag specifies the integer version code for Android and the last
-component of the 1.0.X version for iOS and tvOS.
+The -version flag specifies the semantic version for -buildmode exe. It must
+be on the form major.minor.patch.versioncode where the version code is used for
+the integer version number for Android, iOS and tvOS.
 
 For Android builds the -minsdk flag specify the minimum SDK level. For example,
 use -minsdk 22 to target Android 5.1 (Lollipop) and later.