diff --git a/cmd/dep/ensure.go b/cmd/dep/ensure.go
index 3eabb3482e..9a76fae748 100644
--- a/cmd/dep/ensure.go
+++ b/cmd/dep/ensure.go
@@ -144,8 +144,18 @@ func (cmd *ensureCommand) Run(ctx *dep.Ctx, args []string) error {
 
 	solver, err := gps.Prepare(params, sm)
 	if err != nil {
+		if deduceErrs, ok := err.(gps.DeductionFailureErrs); ok {
+			ctx.Loggers.Err.Println("The following errors occurred while deducing packages:")
+			for ip, error := range deduceErrs {
+				ctx.Loggers.Err.Printf("  * \"%s\": %s", ip, error)
+			}
+			ctx.Loggers.Err.Println()
+
+			return errors.New("could not deduce packages to ensure constraints")
+		}
 		return errors.Wrap(err, "ensure Prepare")
 	}
+
 	solution, err := solver.Solve()
 	if err != nil {
 		handleAllTheFailuresOfTheWorld(err)
diff --git a/internal/gps/solve_failures.go b/internal/gps/solve_failures.go
index e6a2c47a85..43f1ef4864 100644
--- a/internal/gps/solve_failures.go
+++ b/internal/gps/solve_failures.go
@@ -239,6 +239,13 @@ func (e badOptsFailure) Error() string {
 	return string(e)
 }
 
+// DeductionFailureErrs maps package import path to errors occurring during deduction.
+type DeductionFailureErrs map[string]error
+
+func (e DeductionFailureErrs) Error() string {
+	return fmt.Sprintf("%v", e)
+}
+
 type sourceMismatchFailure struct {
 	// The ProjectRoot over which there is disagreement about where it should be
 	// sourced from
diff --git a/internal/gps/solver.go b/internal/gps/solver.go
index 06e6e9afa6..96994476ac 100644
--- a/internal/gps/solver.go
+++ b/internal/gps/solver.go
@@ -10,6 +10,7 @@ import (
 	"log"
 	"sort"
 	"strings"
+	"sync"
 
 	"github.com/armon/go-radix"
 	"github.com/golang/dep/internal/gps/paths"
@@ -289,6 +290,28 @@ func Prepare(params SolveParameters, sm SourceManager) (Solver, error) {
 		rd:       rd,
 	}
 
+	var deducePkgsGroup sync.WaitGroup
+	deductionErrs := make(DeductionFailureErrs)
+
+	checkPkg := func(ip string, sm SourceManager) {
+		_, err := sm.DeduceProjectRoot(ip)
+		if err != nil {
+			deductionErrs[ip] = err
+		}
+		deducePkgsGroup.Done()
+	}
+
+	for _, ip := range rd.externalImportList(params.stdLibFn) {
+		deducePkgsGroup.Add(1)
+		go checkPkg(ip, sm)
+	}
+
+	deducePkgsGroup.Wait()
+
+	if len(deductionErrs) > 0 {
+		return nil, deductionErrs
+	}
+
 	// Set up the bridge and ensure the root dir is in good, working order
 	// before doing anything else.
 	if params.mkBridgeFn == nil {