diff --git a/daemon/errors.go b/daemon/errors.go
index 7282e4f462..803c070f12 100644
--- a/daemon/errors.go
+++ b/daemon/errors.go
@@ -59,19 +59,6 @@ func (e nameConflictError) Error() string {
 
 func (nameConflictError) Conflict() {}
 
-type containerNotModifiedError struct {
-	running bool
-}
-
-func (e containerNotModifiedError) Error() string {
-	if e.running {
-		return "Container is already started"
-	}
-	return "Container is already stopped"
-}
-
-func (e containerNotModifiedError) NotModified() {}
-
 type invalidIdentifier string
 
 func (e invalidIdentifier) Error() string {
diff --git a/daemon/start.go b/daemon/start.go
index 87bb8fba55..110d9c222d 100644
--- a/daemon/start.go
+++ b/daemon/start.go
@@ -14,6 +14,30 @@ import (
 	"github.com/pkg/errors"
 )
 
+// validateState verifies if the container is in a non-conflicting state.
+func validateState(ctr *container.Container) error {
+	ctr.Lock()
+	defer ctr.Unlock()
+
+	// Intentionally checking paused first, because a container can be
+	// BOTH running AND paused. To start a paused (but running) container,
+	// it must be thawed ("un-paused").
+	if ctr.Paused {
+		return errdefs.Conflict(errors.New("cannot start a paused container, try unpause instead"))
+	} else if ctr.Running {
+		// This is not an actual error, but produces a 304 "not modified"
+		// when returned through the API to indicates the container is
+		// already in the desired state. It's implemented as an error
+		// to make the code calling this function terminate early (as
+		// no further processing is needed).
+		return errdefs.NotModified(errors.New("container is already running"))
+	}
+	if ctr.RemovalInProgress || ctr.Dead {
+		return errdefs.Conflict(errors.New("container is marked for removal and cannot be started"))
+	}
+	return nil
+}
+
 // ContainerStart starts a container.
 func (daemon *Daemon) ContainerStart(ctx context.Context, name string, hostConfig *containertypes.HostConfig, checkpoint string, checkpointDir string) error {
 	daemonCfg := daemon.config()
@@ -25,26 +49,7 @@ func (daemon *Daemon) ContainerStart(ctx context.Context, name string, hostConfi
 	if err != nil {
 		return err
 	}
-
-	validateState := func() error {
-		ctr.Lock()
-		defer ctr.Unlock()
-
-		if ctr.Paused {
-			return errdefs.Conflict(errors.New("cannot start a paused container, try unpause instead"))
-		}
-
-		if ctr.Running {
-			return containerNotModifiedError{running: true}
-		}
-
-		if ctr.RemovalInProgress || ctr.Dead {
-			return errdefs.Conflict(errors.New("container is marked for removal and cannot be started"))
-		}
-		return nil
-	}
-
-	if err := validateState(); err != nil {
+	if err := validateState(ctr); err != nil {
 		return err
 	}
 
diff --git a/daemon/stop.go b/daemon/stop.go
index b5b1506d73..4d22a81945 100644
--- a/daemon/stop.go
+++ b/daemon/stop.go
@@ -26,7 +26,12 @@ func (daemon *Daemon) ContainerStop(ctx context.Context, name string, options co
 		return err
 	}
 	if !ctr.IsRunning() {
-		return containerNotModifiedError{}
+		// This is not an actual error, but produces a 304 "not modified"
+		// when returned through the API to indicates the container is
+		// already in the desired state. It's implemented as an error
+		// to make the code calling this function terminate early (as
+		// no further processing is needed).
+		return errdefs.NotModified(errors.New("container is already stopped"))
 	}
 	err = daemon.containerStop(ctx, ctr, options)
 	if err != nil {