diff --git a/CHANGELOG.md b/CHANGELOG.md
index d8d7b4f76a41..a28d052ab49b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -40,6 +40,7 @@ Bug fixes:
 * Fixed File.fnmatch causes ArrayIndexOutOfBoundsException (#1845).
 * Make `String#concat` work with no or multiple arguments (#1519).
 * Make `Array#concat` work with no or multiple arguments (#1519).
+* Fixed BigDecimal coerce initial argument using `to_str` (#1826).
 
 Compatibility:
 
diff --git a/lib/truffle/bigdecimal.rb b/lib/truffle/bigdecimal.rb
index 7f50ca143269..c79a24a7755e 100644
--- a/lib/truffle/bigdecimal.rb
+++ b/lib/truffle/bigdecimal.rb
@@ -326,8 +326,50 @@ def self.new(*args)
 end
 
 module Kernel
-  def BigDecimal(value, precision = Truffle::UNDEFINED)
-    TrufflePrimitive.bigdecimal_new value, precision, true
+  def BigDecimal(value, precision = Truffle::UNDEFINED, exception: true)
+    if !TrufflePrimitive.undefined?(precision)
+      precision = Truffle::Type.rb_num2int(precision)
+      if precision < 0
+        if exception
+          raise ArgumentError, 'negative precision'
+        else
+          return nil
+        end
+      end
+    end
+
+    case value
+    when nil
+      if exception
+        raise TypeError, "can't convert nil into BigDecimal"
+      else
+        return nil
+      end
+    when true
+      if exception
+        raise TypeError, "can't convert true into BigDecimal"
+      else
+        return nil
+      end
+    when false
+      if exception
+        raise TypeError, "can't convert false into BigDecimal"
+      else
+        return nil
+      end
+    when BigDecimal, Integer, Float, Rational, String
+        # conversion handled in primitive
+    else
+      if exception
+        value = StringValue(value)
+      else
+        value = Truffle::Type.rb_check_convert_type(count, String, :to_str)
+        if value.nil?
+          return nil
+        end
+      end
+    end
+    TrufflePrimitive.bigdecimal_new value, precision, exception
   end
 end
 
diff --git a/spec/ruby/library/bigdecimal/BigDecimal_spec.rb b/spec/ruby/library/bigdecimal/BigDecimal_spec.rb
index 179bde1aed45..0c07e46f5907 100644
--- a/spec/ruby/library/bigdecimal/BigDecimal_spec.rb
+++ b/spec/ruby/library/bigdecimal/BigDecimal_spec.rb
@@ -46,6 +46,12 @@
     BigDecimal("  \t\n \r-Infinity   \n").infinite?.should == -1
   end
 
+  it "coerces the value argument with #to_str" do
+    initial = mock("value")
+    initial.should_receive(:to_str).and_return("123")
+    BigDecimal(initial).should == BigDecimal("123")
+  end
+
   ruby_version_is ""..."2.6" do
     it "ignores trailing garbage" do
       BigDecimal("123E45ruby").should == BigDecimal("123E45")