diff --git a/container/docker/handler.go b/container/docker/handler.go
index b9d7c1f6..a5c49e91 100644
--- a/container/docker/handler.go
+++ b/container/docker/handler.go
@@ -227,12 +227,7 @@ func libcontainerConfigToContainerSpec(config *libcontainer.Config, mi *info.Mac
 	if config.Cgroups.CpuShares != 0 {
 		spec.Cpu.Limit = uint64(config.Cgroups.CpuShares)
 	}
-	if config.Cgroups.CpusetCpus == "" {
-		// All cores are active.
-		spec.Cpu.Mask = fmt.Sprintf("0-%d", mi.NumCores-1)
-	} else {
-		spec.Cpu.Mask = config.Cgroups.CpusetCpus
-	}
+	spec.Cpu.Mask = utils.FixCpuMask(config.Cgroups.CpusetCpus, mi.NumCores)
 
 	spec.HasNetwork = true
 	spec.HasDiskIo = true
diff --git a/container/raw/handler.go b/container/raw/handler.go
index 5edfb9b4..155dc95b 100644
--- a/container/raw/handler.go
+++ b/container/raw/handler.go
@@ -217,10 +217,8 @@ func (self *rawContainerHandler) GetSpec() (info.ContainerSpec, error) {
 	if ok {
 		if utils.FileExists(cpusetRoot) {
 			spec.HasCpu = true
-			spec.Cpu.Mask = readString(cpusetRoot, "cpuset.cpus")
-			if spec.Cpu.Mask == "" {
-				spec.Cpu.Mask = fmt.Sprintf("0-%d", mi.NumCores-1)
-			}
+			mask := readString(cpusetRoot, "cpuset.cpus")
+			spec.Cpu.Mask = utils.FixCpuMask(mask, mi.NumCores)
 		}
 	}
 
diff --git a/utils/utils.go b/utils/utils.go
new file mode 100644
index 00000000..9458a4c5
--- /dev/null
+++ b/utils/utils.go
@@ -0,0 +1,29 @@
+// Copyright 2015 Google Inc. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package utils
+
+import "fmt"
+
+// Returns a mask of all cores on the machine if the passed-in mask is empty.
+func FixCpuMask(mask string, cores int) string {
+	if mask == "" {
+		if cores > 1 {
+			mask = fmt.Sprintf("0-%d", cores-1)
+		} else {
+			mask = "0"
+		}
+	}
+	return mask
+}