diff --git a/folly/IPAddressV6.cpp b/folly/IPAddressV6.cpp
index c32d782adb2..4eb043a096f 100644
--- a/folly/IPAddressV6.cpp
+++ b/folly/IPAddressV6.cpp
@@ -230,7 +230,13 @@ IPAddressV6 IPAddressV6::fromInverseArpaName(const std::string& arpaname) {
   size_t pos = 0;
   int count = 0;
   for (size_t i = 1; i <= pieces.size(); i++) {
-    ip[pos] = pieces[pieces.size() - i][0];
+    // Each label is a single hex digit; IPAddressV6 checks that it is hex.
+    auto const label = pieces[pieces.size() - i];
+    if (label.size() != 1) {
+      throw IPAddressFormatException(
+          fmt::format("Invalid input. Got '{}'", piece));
+    }
+    ip[pos] = label[0];
     pos++;
     count++;
     // add ':' every 4 chars
diff --git a/folly/test/IPAddressTest.cpp b/folly/test/IPAddressTest.cpp
index a6ac55739ae..7c46a4a469a 100644
--- a/folly/test/IPAddressTest.cpp
+++ b/folly/test/IPAddressTest.cpp
@@ -776,6 +776,19 @@ TEST(IPaddress, fromInverseArpaName) {
           fmt::format(
               "{}.ip6.arpa",
               "3.0.0.0.0.0.0.0.0.0.0.0.c.0.0.b.e.c.a.f.e.f.c.1.0.0.0.0.0.2.6.2")));
+  // Each label must be exactly one hex digit.
+  EXPECT_THROW(
+      IPAddressV6::fromInverseArpaName(
+          fmt::format(
+              "{}.ip6.arpa",
+              "30.0.0.0.0.0.0.0.0.0.0.0.c.0.0.b.e.c.a.f.e.f.c.1.0.0.0.0.0.2.6.2")),
+      IPAddressFormatException);
+  EXPECT_THROW(
+      IPAddressV6::fromInverseArpaName(
+          fmt::format(
+              "{}.ip6.arpa",
+              ".0.0.0.0.0.0.0.0.0.0.0.c.0.0.b.e.c.a.f.e.f.c.1.0.0.0.0.0.2.6.2")),
+      IPAddressFormatException);
 }
 
 // Test that invalid string values are killed