diff --git a/android/api_levels.go b/android/api_levels.go
index 9440ee9..a68d8e2 100644
--- a/android/api_levels.go
+++ b/android/api_levels.go
@@ -55,6 +55,9 @@
 }
 
 func (this ApiLevel) FinalInt() int {
+	if this.IsInvalid() {
+		panic(fmt.Errorf("%v is not a recognized api_level\n", this))
+	}
 	if this.IsPreview() {
 		panic("Requested a final int from a non-final ApiLevel")
 	} else {
@@ -63,6 +66,9 @@
 }
 
 func (this ApiLevel) FinalOrFutureInt() int {
+	if this.IsInvalid() {
+		panic(fmt.Errorf("%v is not a recognized api_level\n", this))
+	}
 	if this.IsPreview() {
 		return FutureApiLevelInt
 	} else {
@@ -76,6 +82,9 @@
 // - preview codenames -> preview base (9000) + index
 // - otherwise -> cast to int
 func (this ApiLevel) FinalOrPreviewInt() int {
+	if this.IsInvalid() {
+		panic(fmt.Errorf("%v is not a recognized api_level\n", this))
+	}
 	if this.IsCurrent() {
 		return this.number
 	}
@@ -97,6 +106,11 @@
 	return this.isPreview
 }
 
+// Returns true if the raw api level string is invalid
+func (this ApiLevel) IsInvalid() bool {
+	return this.EqualTo(InvalidApiLevel)
+}
+
 // Returns true if this is the unfinalized "current" API level. This means
 // different things across Java and native. Java APIs do not use explicit
 // codenames, so all non-final codenames are grouped into "current". For native
@@ -113,6 +127,12 @@
 	return this.number == -1
 }
 
+// Returns true if an app is compiling against private apis.
+// e.g. if sdk_version = "" in Android.bp, then the ApiLevel of that "sdk" is at PrivateApiLevel.
+func (this ApiLevel) IsPrivate() bool {
+	return this.number == PrivateApiLevel.number
+}
+
 // Returns -1 if the current API level is less than the argument, 0 if they
 // are equal, and 1 if it is greater than the argument.
 func (this ApiLevel) CompareTo(other ApiLevel) int {
@@ -166,6 +186,19 @@
 	isPreview: true,
 }
 
+// Sentinel ApiLevel to validate that an apiLevel is either an int or a recognized codename.
+var InvalidApiLevel = NewInvalidApiLevel("invalid")
+
+// Returns an apiLevel object at the same level as InvalidApiLevel.
+// The object contains the raw string provied in bp file, and can be used for error handling.
+func NewInvalidApiLevel(raw string) ApiLevel {
+	return ApiLevel{
+		value:     raw,
+		number:    -2, // One less than NoneApiLevel
+		isPreview: true,
+	}
+}
+
 // The first version that introduced 64-bit ABIs.
 var FirstLp64Version = uncheckedFinalApiLevel(21)
 
diff --git a/android/config.go b/android/config.go
index 292fcf2..593812c 100644
--- a/android/config.go
+++ b/android/config.go
@@ -52,6 +52,15 @@
 // FutureApiLevelInt is a placeholder constant for unreleased API levels.
 const FutureApiLevelInt = 10000
 
+// PrivateApiLevel represents the api level of SdkSpecPrivate (sdk_version: "")
+// This api_level exists to differentiate user-provided "" from "current" sdk_version
+// The differentiation is necessary to enable different validation rules for these two possible values.
+var PrivateApiLevel = ApiLevel{
+	value:     "current",             // The value is current since aidl expects `current` as the default (TestAidlFlagsWithMinSdkVersion)
+	number:    FutureApiLevelInt + 1, // This is used to differentiate it from FutureApiLevel
+	isPreview: true,
+}
+
 // FutureApiLevel represents unreleased API levels.
 var FutureApiLevel = ApiLevel{
 	value:     "current",
diff --git a/android/sdk_version.go b/android/sdk_version.go
index cace88a..8925bba 100644
--- a/android/sdk_version.go
+++ b/android/sdk_version.go
@@ -272,7 +272,7 @@
 
 var (
 	SdkSpecNone         = SdkSpec{SdkNone, NoneApiLevel, "(no version)"}
-	SdkSpecPrivate      = SdkSpec{SdkPrivate, FutureApiLevel, ""}
+	SdkSpecPrivate      = SdkSpec{SdkPrivate, PrivateApiLevel, ""}
 	SdkSpecCorePlatform = SdkSpec{SdkCorePlatform, FutureApiLevel, "core_platform"}
 )
 
@@ -295,7 +295,7 @@
 
 		var kindString string
 		if sep == 0 {
-			return SdkSpec{SdkInvalid, NoneApiLevel, str}
+			return SdkSpec{SdkInvalid, NewInvalidApiLevel(str), str}
 		} else if sep == -1 {
 			kindString = ""
 		} else {
@@ -323,7 +323,7 @@
 
 		apiLevel, err := ApiLevelFromUserWithConfig(config, versionString)
 		if err != nil {
-			return SdkSpec{SdkInvalid, apiLevel, str}
+			return SdkSpec{SdkInvalid, NewInvalidApiLevel(versionString), str}
 		}
 		return SdkSpec{kind, apiLevel, str}
 	}
diff --git a/android/sdk_version_test.go b/android/sdk_version_test.go
index ec81782..ea99c4d 100644
--- a/android/sdk_version_test.go
+++ b/android/sdk_version_test.go
@@ -37,11 +37,11 @@
 		},
 		{
 			input:    "_",
-			expected: "invalid_(no version)",
+			expected: "invalid__",
 		},
 		{
 			input:    "_31",
-			expected: "invalid_(no version)",
+			expected: "invalid__31",
 		},
 		{
 			input:    "system_R",