diff --git a/lib/reline/terminfo.rb b/lib/reline/terminfo.rb
index 95ff082172..b053ba82c4 100644
--- a/lib/reline/terminfo.rb
+++ b/lib/reline/terminfo.rb
@@ -121,6 +121,7 @@ module Reline::Terminfo
   end
 
   def self.tigetstr(capname)
+    raise TerminfoError, "capname is not String: #{capname.inspect}" unless capname.is_a?(String)
     capability = @tigetstr.(capname)
     case capability.to_i
     when 0, -1
@@ -138,6 +139,7 @@ module Reline::Terminfo
   end
 
   def self.tigetflag(capname)
+    raise TerminfoError, "capname is not String: #{capname.inspect}" unless capname.is_a?(String)
     flag = @tigetflag.(capname).to_i
     case flag
     when -1
@@ -149,6 +151,7 @@ module Reline::Terminfo
   end
 
   def self.tigetnum(capname)
+    raise TerminfoError, "capname is not String: #{capname.inspect}" unless capname.is_a?(String)
     num = @tigetnum.(capname).to_i
     case num
     when -2
diff --git a/test/reline/test_terminfo.rb b/test/reline/test_terminfo.rb
index b969da1397..dda9b32495 100644
--- a/test/reline/test_terminfo.rb
+++ b/test/reline/test_terminfo.rb
@@ -14,6 +14,7 @@ class Reline::Terminfo::Test < Reline::TestCase
 
   def test_tigetstr_with_error
     assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetstr('unknown') }
+    assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetstr(nil) }
   end
 
   def test_tiparm
@@ -43,6 +44,7 @@ class Reline::Terminfo::Test < Reline::TestCase
   def test_tigetflag_with_error
     assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetflag('cuu') }
     assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetflag('unknown') }
+    assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetflag(nil) }
   end
 
   def test_tigetnum
@@ -54,5 +56,6 @@ class Reline::Terminfo::Test < Reline::TestCase
   def test_tigetnum_with_error
     assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetnum('cuu') }
     assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetnum('unknown') }
+    assert_raise(Reline::Terminfo::TerminfoError) { Reline::Terminfo.tigetnum(nil) }
   end
 end if Reline::Terminfo.enabled?