diff --git a/src/main/kotlin/org/wfanet/measurement/kingdom/service/api/v2alpha/ProtoConversions.kt b/src/main/kotlin/org/wfanet/measurement/kingdom/service/api/v2alpha/ProtoConversions.kt
index f6e91e315ac..94e1cb9ad5a 100644
--- a/src/main/kotlin/org/wfanet/measurement/kingdom/service/api/v2alpha/ProtoConversions.kt
+++ b/src/main/kotlin/org/wfanet/measurement/kingdom/service/api/v2alpha/ProtoConversions.kt
@@ -140,8 +140,8 @@ fun InternalNoiseMechanism.toNoiseMechanism(): NoiseMechanism {
   return when (this) {
     InternalNoiseMechanism.GEOMETRIC -> NoiseMechanism.GEOMETRIC
     InternalNoiseMechanism.DISCRETE_GAUSSIAN -> NoiseMechanism.DISCRETE_GAUSSIAN
-    InternalNoiseMechanism.UNRECOGNIZED,
-    InternalNoiseMechanism.NOISE_MECHANISM_UNSPECIFIED -> error("Noise mechanism not specified")
+    InternalNoiseMechanism.NOISE_MECHANISM_UNSPECIFIED,
+    InternalNoiseMechanism.UNRECOGNIZED -> error("invalid internal noise mechanism.")
   }
 }
 
@@ -194,7 +194,16 @@ fun InternalProtocolConfig.toProtocolConfig(
                   }
                   ellipticCurveId = source.liquidLegionsV2.ellipticCurveId
                   maximumFrequency = source.liquidLegionsV2.maximumFrequency
-                  noiseMechanism = source.liquidLegionsV2.noiseMechanism.toNoiseMechanism()
+                  // Use `GEOMETRIC` for unspecified InternalNoiseMechanism for old Measurements.
+                  noiseMechanism =
+                    if (
+                      source.liquidLegionsV2.noiseMechanism ==
+                        InternalNoiseMechanism.NOISE_MECHANISM_UNSPECIFIED
+                    ) {
+                      NoiseMechanism.GEOMETRIC
+                    } else {
+                      source.liquidLegionsV2.noiseMechanism.toNoiseMechanism()
+                    }
                 }
               }
             }
diff --git a/src/main/kotlin/org/wfanet/measurement/kingdom/service/system/v1alpha/ProtoConversions.kt b/src/main/kotlin/org/wfanet/measurement/kingdom/service/system/v1alpha/ProtoConversions.kt
index a18994b9b0a..40b647c3c6b 100644
--- a/src/main/kotlin/org/wfanet/measurement/kingdom/service/system/v1alpha/ProtoConversions.kt
+++ b/src/main/kotlin/org/wfanet/measurement/kingdom/service/system/v1alpha/ProtoConversions.kt
@@ -230,7 +230,16 @@ fun buildMpcProtocolConfig(
           }
           ellipticCurveId = protocolConfig.liquidLegionsV2.ellipticCurveId
           maximumFrequency = protocolConfig.liquidLegionsV2.maximumFrequency
-          noiseMechanism = protocolConfig.liquidLegionsV2.noiseMechanism.toSystemNoiseMechanism()
+          // Use `GEOMETRIC` for unspecified InternalNoiseMechanism for old Measurements.
+          noiseMechanism =
+            if (
+              protocolConfig.liquidLegionsV2.noiseMechanism ==
+                InternalProtocolConfig.NoiseMechanism.NOISE_MECHANISM_UNSPECIFIED
+            ) {
+              NoiseMechanism.GEOMETRIC
+            } else {
+              protocolConfig.liquidLegionsV2.noiseMechanism.toSystemNoiseMechanism()
+            }
         }
       }
     }
@@ -336,12 +345,12 @@ fun DuchyMeasurementLogEntry.toSystemComputationLogEntry(
     .build()
 }
 
-/** Converts an internal NoiseMechanism to a system NoiseMechanism */
+/** Converts an internal NoiseMechanism to a system NoiseMechanism. */
 fun InternalNoiseMechanism.toSystemNoiseMechanism(): NoiseMechanism {
   return when (this) {
     InternalNoiseMechanism.GEOMETRIC -> NoiseMechanism.GEOMETRIC
     InternalNoiseMechanism.DISCRETE_GAUSSIAN -> NoiseMechanism.DISCRETE_GAUSSIAN
-    InternalNoiseMechanism.UNRECOGNIZED,
-    InternalNoiseMechanism.NOISE_MECHANISM_UNSPECIFIED -> error("Invalid internal NoiseMechanism.")
+    InternalNoiseMechanism.NOISE_MECHANISM_UNSPECIFIED,
+    InternalNoiseMechanism.UNRECOGNIZED -> error("invalid internal noise mechanism.")
   }
 }
diff --git a/src/main/proto/wfa/measurement/internal/kingdom/protocol_config.proto b/src/main/proto/wfa/measurement/internal/kingdom/protocol_config.proto
index cf34e9501f0..b5c954e90c8 100644
--- a/src/main/proto/wfa/measurement/internal/kingdom/protocol_config.proto
+++ b/src/main/proto/wfa/measurement/internal/kingdom/protocol_config.proto
@@ -55,6 +55,8 @@ message ProtocolConfig {
     int32 maximum_frequency = 4;
 
     // The mechanism to generate noise during computation.
+    //
+    // If not specified the value is assumed to be `GEOMETRIC`.
     NoiseMechanism noise_mechanism = 5;
   }