diff --git a/cmd/d2/main.go b/cmd/d2/main.go
index d5976671a..e111d72f0 100644
--- a/cmd/d2/main.go
+++ b/cmd/d2/main.go
@@ -31,12 +31,27 @@ func run(ctx context.Context, ms *xmain.State) (err error) {
 	// :(
 	ctx = xmain.DiscardSlog(ctx)
 
-	watchFlag := ms.Opts.Bool("D2_WATCH", "watch", "w", false, "watch for changes to input and live reload. Use $HOST and $PORT to specify the listening address.\n$D2_HOST and $D2_PORT are also accepted and take priority (default localhost:0, which is will open on a randomly available local port).")
-	bundleFlag := ms.Opts.Bool("D2_BUNDLE", "bundle", "b", true, "bundle all assets and layers into the output svg.")
-	debugFlag := ms.Opts.Bool("DEBUG", "debug", "d", false, "print debug logs.")
+	watchFlag, err := ms.Opts.Bool("D2_WATCH", "watch", "w", false, "watch for changes to input and live reload. Use $HOST and $PORT to specify the listening address.\n$D2_HOST and $D2_PORT are also accepted and take priority (default localhost:0, which is will open on a randomly available local port).")
+	if err != nil {
+		return xmain.UsageErrorf(err.Error())
+	}
+	bundleFlag, err := ms.Opts.Bool("D2_BUNDLE", "bundle", "b", true, "bundle all assets and layers into the output svg.")
+	if err != nil {
+		return xmain.UsageErrorf(err.Error())
+	}
+	debugFlag, err := ms.Opts.Bool("DEBUG", "debug", "d", false, "print debug logs.")
+	if err != nil {
+		return xmain.UsageErrorf(err.Error())
+	}
 	layoutFlag := ms.Opts.String("D2_LAYOUT", "layout", "l", "dagre", `the layout engine used.`)
-	themeFlag := ms.Opts.Int64("D2_THEME", "theme", "t", 0, "the diagram theme ID. For a list of available options, see https://oss.terrastruct.com/d2")
-	versionFlag := ms.Opts.Bool("", "version", "v", false, "get the version")
+	themeFlag, err := ms.Opts.Int64("D2_THEME", "theme", "t", 0, "the diagram theme ID. For a list of available options, see https://oss.terrastruct.com/d2")
+	if err != nil {
+		return xmain.UsageErrorf(err.Error())
+	}
+	versionFlag, err := ms.Opts.Bool("", "version", "v", false, "get the version")
+	if err != nil {
+		return xmain.UsageErrorf(err.Error())
+	}
 
 	err = ms.Opts.Parse()
 	if !errors.Is(err, pflag.ErrHelp) && err != nil {
diff --git a/lib/xmain/opts.go b/lib/xmain/opts.go
index c095d58cb..15a0c71a1 100644
--- a/lib/xmain/opts.go
+++ b/lib/xmain/opts.go
@@ -20,7 +20,7 @@ type Opts struct {
 	registeredEnvs []string
 }
 
-func NewOpts(env *xos.Env, args []string, log *cmdlog.Logger) *Opts {
+func NewOpts(env *xos.Env, log *cmdlog.Logger, args []string) *Opts {
 	flags := pflag.NewFlagSet("", pflag.ContinueOnError)
 	flags.SortFlags = false
 	flags.Usage = func() {}
@@ -52,29 +52,26 @@ func (o *Opts) Help() string {
 	return b.String()
 }
 
-func (o *Opts) Int64(envKey, flag, shortFlag string, defaultVal int64, usage string) *int64 {
+func (o *Opts) Int64(envKey, flag, shortFlag string, defaultVal int64, usage string) (*int64, error) {
 	if envKey != "" {
 		if o.env.Getenv(envKey) != "" {
 			envVal, err := strconv.ParseInt(o.env.Getenv(envKey), 10, 64)
 			if err != nil {
-				o.log.Error.Printf(`ignoring invalid environment variable %s. Expected int64. Found "%v".`, envKey, envVal)
-			} else if envVal != defaultVal {
-				defaultVal = envVal
+				return nil, fmt.Errorf(`invalid environment variable %s. Expected int64. Found "%v".`, envKey, envVal)
 			}
+			defaultVal = envVal
 		}
 		o.registeredEnvs = append(o.registeredEnvs, envKey)
 	}
 
-	return o.flags.Int64P(flag, shortFlag, defaultVal, usage)
+	return o.flags.Int64P(flag, shortFlag, defaultVal, usage), nil
 }
 
 func (o *Opts) String(envKey, flag, shortFlag string, defaultVal, usage string) *string {
 	if envKey != "" {
 		if o.env.Getenv(envKey) != "" {
 			envVal := o.env.Getenv(envKey)
-			if envVal != defaultVal {
-				defaultVal = envVal
-			}
+			defaultVal = envVal
 		}
 		o.registeredEnvs = append(o.registeredEnvs, envKey)
 	}
@@ -82,21 +79,23 @@ func (o *Opts) String(envKey, flag, shortFlag string, defaultVal, usage string)
 	return o.flags.StringP(flag, shortFlag, defaultVal, usage)
 }
 
-func (o *Opts) Bool(envKey, flag, shortFlag string, defaultVal bool, usage string) *bool {
+func (o *Opts) Bool(envKey, flag, shortFlag string, defaultVal bool, usage string) (*bool, error) {
 	if envKey != "" {
 		if o.env.Getenv(envKey) != "" {
 			envVal := o.env.Getenv(envKey)
 			if !boolyEnv(envVal) {
-				o.log.Error.Printf(`ignoring invalid environment variable %s. Expected bool. Found "%s".`, envKey, envVal)
-			} else if (defaultVal && falseyEnv(envVal)) ||
-				(!defaultVal && truthyEnv(envVal)) {
-				defaultVal = !defaultVal
+				return nil, fmt.Errorf(`invalid environment variable %s. Expected bool. Found "%s".`, envKey, envVal)
+			}
+			if truthyEnv(envVal) {
+				defaultVal = true
+			} else {
+				defaultVal = false
 			}
 		}
 		o.registeredEnvs = append(o.registeredEnvs, envKey)
 	}
 
-	return o.flags.BoolP(flag, shortFlag, defaultVal, usage)
+	return o.flags.BoolP(flag, shortFlag, defaultVal, usage), nil
 }
 
 func boolyEnv(s string) bool {
@@ -104,19 +103,15 @@ func boolyEnv(s string) bool {
 }
 
 func falseyEnv(s string) bool {
-	return s == "0" || s == "false" || s == "f"
+	return s == "0" || s == "false"
 }
 
 func truthyEnv(s string) bool {
-	return s == "1" || s == "true" || s == "t"
+	return s == "1" || s == "true"
 }
 
 func (o *Opts) Parse() error {
-	err := o.flags.Parse(o.args)
-	if err != nil {
-		return err
-	}
-	return nil
+	return o.flags.Parse(o.args)
 }
 
 func (o *Opts) SetArgs(args []string) {
diff --git a/lib/xmain/xmain.go b/lib/xmain/xmain.go
index 3ef9b89aa..18eba1367 100644
--- a/lib/xmain/xmain.go
+++ b/lib/xmain/xmain.go
@@ -42,7 +42,7 @@ func Main(run RunFunc) {
 		Env: xos.NewEnv(os.Environ()),
 	}
 	ms.Log = cmdlog.Log(ms.Env, os.Stderr)
-	ms.Opts = NewOpts(ms.Env, args, ms.Log)
+	ms.Opts = NewOpts(ms.Env, ms.Log, args)
 
 	sigs := make(chan os.Signal, 1)
 	signal.Notify(sigs, os.Interrupt, syscall.SIGTERM)