diff --git a/API-CHANGELOG.md b/API-CHANGELOG.md
index 79fb8ff522..bc3672588e 100644
--- a/API-CHANGELOG.md
+++ b/API-CHANGELOG.md
@@ -54,6 +54,7 @@ This section contains changes targeting a future version.
 - `submit`: The `fail_hard` field now returns an error if the value is not a boolean. [#6529](https://github.com/XRPLF/rippled/pull/6529)
 - `subscribe`: The `taker` field in the `books` array now returns `actMalformed` instead of `badIssuer` if the value is not a valid account. [#6529](https://github.com/XRPLF/rippled/pull/6529)
 - Fixed a bug in `Forwarded` HTTP header parsing where the extracted IP address could be incorrect when no comma or semicolon delimiter follows the address. This could cause the server to misidentify a client's IP address when operating behind a reverse proxy. [#6529](https://github.com/XRPLF/rippled/pull/6529)
+- `account_lines`: The `peer` field now returns an error if the value is not a string. [#7728](https://github.com/XRPLF/rippled/pull/7728)
 
 ## XRP Ledger server version 3.1.0
 
diff --git a/src/test/rpc/AccountLines_test.cpp b/src/test/rpc/AccountLines_test.cpp
index cb20de9bf5..3de2bdefa3 100644
--- a/src/test/rpc/AccountLines_test.cpp
+++ b/src/test/rpc/AccountLines_test.cpp
@@ -94,6 +94,24 @@ public:
         LedgerHeader const ledger3Info = env.closed()->header();
         BEAST_EXPECT(ledger3Info.seq == 3);
 
+        {
+            // test peer non-string
+            auto testInvalidPeerParam = [&](auto const& param) {
+                json::Value params;
+                params[jss::account] = alice.human();
+                params[jss::peer] = param;
+                auto jrr = env.rpc("json", "account_lines", to_string(params))[jss::result];
+                BEAST_EXPECT(jrr[jss::error] == "invalidParams");
+                BEAST_EXPECT(jrr[jss::error_message] == "Invalid field 'peer'.");
+            };
+
+            testInvalidPeerParam(1);
+            testInvalidPeerParam(1.1);
+            testInvalidPeerParam(true);
+            testInvalidPeerParam(json::Value(json::ValueType::Null));
+            testInvalidPeerParam(json::Value(json::ValueType::Object));
+            testInvalidPeerParam(json::Value(json::ValueType::Array));
+        }
         {
             // alice is funded but has no lines.  An empty array is returned.
             json::Value params;
@@ -775,6 +793,35 @@ public:
         LedgerHeader const ledger3Info = env.closed()->header();
         BEAST_EXPECT(ledger3Info.seq == 3);
 
+        {
+            // test peer non-string
+            auto testInvalidPeerParam = [&](auto const& param) {
+                json::Value params;
+                params[jss::account] = alice.human();
+                params[jss::peer] = param;
+
+                json::Value request;
+                request[jss::method] = "account_lines";
+                request[jss::jsonrpc] = "2.0";
+                request[jss::ripplerpc] = "2.0";
+                request[jss::id] = 5;
+                request[jss::params] = params;
+
+                auto const lines = env.rpc("json2", to_string(request));
+                BEAST_EXPECT(lines[jss::error][jss::error] == "invalidParams");
+                BEAST_EXPECT(lines[jss::error][jss::message] == "Invalid field 'peer'.");
+                BEAST_EXPECT(lines.isMember(jss::jsonrpc) && lines[jss::jsonrpc] == "2.0");
+                BEAST_EXPECT(lines.isMember(jss::ripplerpc) && lines[jss::ripplerpc] == "2.0");
+                BEAST_EXPECT(lines.isMember(jss::id) && lines[jss::id] == 5);
+            };
+
+            testInvalidPeerParam(1);
+            testInvalidPeerParam(1.1);
+            testInvalidPeerParam(true);
+            testInvalidPeerParam(json::Value(json::ValueType::Null));
+            testInvalidPeerParam(json::Value(json::ValueType::Object));
+            testInvalidPeerParam(json::Value(json::ValueType::Array));
+        }
         {
             // alice is funded but has no lines.  An empty array is returned.
             json::Value params;
diff --git a/src/xrpld/rpc/handlers/account/AccountLines.cpp b/src/xrpld/rpc/handlers/account/AccountLines.cpp
index 4a6d22d5d8..ac98e271b6 100644
--- a/src/xrpld/rpc/handlers/account/AccountLines.cpp
+++ b/src/xrpld/rpc/handlers/account/AccountLines.cpp
@@ -107,7 +107,12 @@ doAccountLines(rpc::JsonContext& context)
 
     std::string strPeer;
     if (params.isMember(jss::peer))
+    {
+        if (!params[jss::peer].isString())
+            return rpc::invalidFieldError(jss::peer);
+
         strPeer = params[jss::peer].asString();
+    }
 
     auto const raPeerAccount = [&]() -> std::optional<AccountID> {
         return strPeer.empty() ? std::nullopt : parseBase58<AccountID>(strPeer);