diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0534e71d7..43b028938 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
   - This applies to `submit`, `signAndSubmit`, `restoreFootprint`, and therefore to `ContractClient.invoke` when it submits.
 - fix: always encode text as UTF-8 in `Scv.toString(String)`, `Sep10Challenge` (`web_auth_domain` / `client_domain`), `Util.paddedByteArray(String, int)`, and the `SorobanServer` request body. They used the JVM default charset, so non-ASCII text produced different bytes on JVMs where that is not UTF-8.
 - chore: upgrade generated XDR definitions to Protocol 30.
+- fix: `StrKey.decodeClaimableBalance` and `StrKey.isValidClaimableBalance` now reject a claimable balance strkey (`B...`) whose type byte is not 0, as SEP-23 requires; `CLAIMABLE_BALANCE_ID_TYPE_V0` is the only type. Such a key used to be accepted, so `new Address(...)` took it and the error only surfaced in `Address.toSCAddress`. `new Address(...)` now throws `IllegalArgumentException` up front.
 - refactor: deprecate `GetEventsResponse.EventInfo.inSuccessfulContractCall`. Stellar RPC no longer returns it from `getEvents` ([stellar-rpc#1050](https://github.com/stellar/stellar-rpc/pull/1050)), so it is `null` with newer servers. It will be removed in the next major release.
 
 ## 5.0.0
diff --git a/src/main/java/org/stellar/sdk/StrKey.java b/src/main/java/org/stellar/sdk/StrKey.java
index b6ed2a625..1e97dfd91 100644
--- a/src/main/java/org/stellar/sdk/StrKey.java
+++ b/src/main/java/org/stellar/sdk/StrKey.java
@@ -454,6 +454,13 @@ static byte[] decodeCheck(VersionByte versionByte, char[] encoded) {
           throw new IllegalArgumentException(
               "Invalid data length, expected 33 bytes, got " + data.length);
         }
+        // SEP-23: the first byte is the claimable balance type, and CLAIMABLE_BALANCE_ID_TYPE_V0
+        // is the only one.
+        if (data[0] != 0) {
+          throw new IllegalArgumentException(
+              "Invalid claimable balance type, expected 0 (CLAIMABLE_BALANCE_ID_TYPE_V0), got "
+                  + data[0]);
+        }
         break;
       default:
         if (data.length != 32) {
diff --git a/src/test/kotlin/org/stellar/sdk/AddressTest.kt b/src/test/kotlin/org/stellar/sdk/AddressTest.kt
index c68acadf9..c2bacb866 100644
--- a/src/test/kotlin/org/stellar/sdk/AddressTest.kt
+++ b/src/test/kotlin/org/stellar/sdk/AddressTest.kt
@@ -50,6 +50,14 @@ class AddressTest :
         address.addressType shouldBe Address.AddressType.LIQUIDITY_POOL
       }
 
+      test("should throw exception for claimable balance ID with a non-zero type") {
+        val exception =
+          shouldThrow<IllegalArgumentException> {
+            Address("BAAT6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGXACA")
+          }
+        exception.message shouldBe "Unsupported address type"
+      }
+
       test("should throw exception for invalid address") {
         val exception = shouldThrow<IllegalArgumentException> { Address(invalidAddress) }
         exception.message shouldBe "Unsupported address type"
diff --git a/src/test/kotlin/org/stellar/sdk/StrKeyTest.kt b/src/test/kotlin/org/stellar/sdk/StrKeyTest.kt
index e8e303c30..b7b34ba47 100644
--- a/src/test/kotlin/org/stellar/sdk/StrKeyTest.kt
+++ b/src/test/kotlin/org/stellar/sdk/StrKeyTest.kt
@@ -191,6 +191,21 @@ class StrKeyTest :
         StrKey.isValidClaimableBalance(
           "BAT7RRVQBTQ3TEDLDICV7GNX6GE3KVQFJCNOESUI4QNA4TYYR44P2FS5PZYQ"
         ) shouldBe false
+        // SEP-23: invalid claimable balance type (first byte of binary key is not 0)
+        StrKey.isValidClaimableBalance(
+          "BAAT6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGXACA"
+        ) shouldBe false
+      }
+
+      test("decode rejects a non-zero claimable balance type") {
+        val exception =
+          shouldThrow<IllegalArgumentException> {
+            StrKey.decodeClaimableBalance(
+              "BAAT6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGXACA"
+            )
+          }
+        exception.message shouldBe
+          "Invalid claimable balance type, expected 0 (CLAIMABLE_BALANCE_ID_TYPE_V0), got 1"
       }
     }
 
@@ -355,9 +370,8 @@ class StrKeyTest :
         "PA7QYNF7SOWQ3GLR2BGMZEHXAVIRZA4KVWLTJJFC7MGXUA74P7UJUAAAAAOQCAQDAQCQMBYIBEFAWDANBYHRAEISCMKBKFQXDAMRUGY4Z2PQ",
         "PA7QYNF7SOWQ3GLR2BGMZEHXAVIRZA4KVWLTJJFC7MGXUA74P7UJUAAAAAOQCAQDAQCQMBYIBEFAWDANBYHRAEISCMKBKFQXDAMRUGY4DXFH6",
         "BAAD6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGR4TV",
-        // We should consider this valid
-        // Invalid claimablebalance type (first byte of binary key is not 0),
-        // "BAAT6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGXACA"
+        // Invalid claimable balance type (first byte of binary key is not 0)
+        "BAAT6DBUX6J22DMZOHIEZTEQ64CVCHEDRKWZONFEUL5Q26QD7R76RGXACA",
         "",
       ) { key ->
         shouldThrow<IllegalArgumentException> { StrKey.decodeEd25519PublicKey(key) }