diff --git a/cmd/divisor.go b/cmd/divisor.go
index 4a00a1f..c64dd0b 100644
--- a/cmd/divisor.go
+++ b/cmd/divisor.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -47,7 +46,8 @@ func divisorSum(cmd *cobra.Command, args []string) {
 	for _, arg := range args {
 		n, ok := new(big.Int).SetString(arg, 10)
 		if !ok {
-			log.Print("invalid input " + arg)
+			cmd.PrintErrln("invalid input " + arg)
+			continue
 		}
 
 		d := divisorSummatory(n)
diff --git a/cmd/isprime.go b/cmd/isprime.go
index 283b86c..bfd832d 100644
--- a/cmd/isprime.go
+++ b/cmd/isprime.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -32,7 +31,7 @@ func isprime(cmd *cobra.Command, args []string) {
 	for _, s := range args {
 		_, ok := x.SetString(s, 10)
 		if !ok {
-			log.Print("invalid input " + s)
+			cmd.PrintErrln("invalid input " + s)
 			continue
 		}
 
diff --git a/cmd/sqrtRepetend.go b/cmd/sqrtRepetend.go
index 62c73b3..3d72bdb 100644
--- a/cmd/sqrtRepetend.go
+++ b/cmd/sqrtRepetend.go
@@ -18,7 +18,6 @@ package cmd
 
 import (
 	"fmt"
-	"log"
 	"math/big"
 
 	"github.com/spf13/cobra"
@@ -31,13 +30,13 @@ func sqrtRepetend(cmd *cobra.Command, args []string) {
 	for _, s := range args {
 		_, ok := x.SetString(s, 10)
 		if !ok {
-			log.Print("invalid input " + s)
+			cmd.PrintErrln("invalid input " + s)
 			continue
 		}
 
 		repetend, err := lib.SqrtRepetend(x)
 		if err != nil {
-			log.Print(s + " is a perfect square")
+			cmd.PrintErrln(s + " is a perfect square")
 			continue
 		}